Baseline weights are according to the top answer to https://scifi.stackexchange.com/questions/147000/what-is-the-weight-of-ponies-of-the-my-little-pony-universe , which I use as a standard basis for average weights for adult ponies in my writing. As far as I’m aware, no such discussion is available to apply weight estimates to Alicorns, foals, fillies and colts, or other outlier groups or other MLP species, so it’s all a guessing game. Characters mentioned as examples are depicted as their canon sizes and weights.
Earth Ponies
250-350 pounds normal weight
While Applejack kicking a multi-ton boulder an exceptional distance away during the search for the Pillars’ artifacts could be explained away as a power buff given to the Elements, examples such as Big Mac in general and Maud Pie’s introduction episode cement into fact this subspecies’ innate physical prowess. It stands to reason that Earth Ponies would handle excess weight the best of all the subspecies, up to a point. The physical strength allows this type of pony to remain mobile and comparably unhindered by excess weight in respect to the other subspecies. Earth ponies are still subject to the expected physics and pitfalls of size, from issues fitting through doorways and into furniture to a sufficiently large belly impacting walking leg movements. Even a lazy, unexercised Earth Pony would be able to walk across Canterlot without significant fatigue while boasting a BMI of 40-50.
Where Earth Ponies will fall behind, however, are at the point of near-immobility and the point of immobility. In the former, their strength still gives them a slight edge, but in the latter they’re on equal footing to Pegasi and lose to Unicorns. In total immobility, some device or assistant is required for acquisition of food and other bodily needs. Total immobility is generally reached only when the hooves of the pony in question are no longer able to reach the floor. In some cases, very limited mobility is maintained so long as one set of hooves can reach the floor.

Pegasi
125-225 pounds normal weight
Despite exaggerated examples like the gag character Bulk Biceps and a similar character during one of the buckball episodes, Pegasi generally have moderate physical strength with lean muscle, and presumably either some localized magic or Equestrian physics differing from Earth physics used to fly with relatively small wings.  While at a physical advantage to Unicorns on the ground, which is the only advantage a Pegasus has over any subspecies as soon as flight is impossible. Bulk Biceps and that unnamed(?) muscular mare from the buckball episode are collectively an odd outlier, so for the purposes of sensibility regarding flight capabilities and not having 1000 pound Pegasi flying around, we will consider the following:
A.	Bulk Biceps and Pegasi characters with similar builds are not actually as exaggeratedly large as they are depicted in the show. They are larger and bulkier than the average Pegasus and may even exceed the average weight range, but not to the extent depicted in canon media. This allows them to remain in flight, but at a decreased speed and agility compared to a more standard Pegasus. OR:
B.	The exaggerated forms remain, but flight abilities are minimal to non-existent.  Ponies of this nature are even bigger than fat gag character Whoa Nelly, and muscle weighs more than fat.
As seen in canon media, Pegasi are strong enough to carry other ponies, including the heavier Earth Ponies, in flight without considerable strain. Their speed is considerably limited and they aren’t show making long-distance flights. However, it can be surmised that a Pegasus can maintain flight capabilities at a diminished rate at their current weight + up to 350 pounds, though I cannot personally recall seeing an earth pony on the larger side being carried by a Pegasus, just smaller ones such as Pinkie Pie and AppleJack.  Larger, heavier characters such as Big Mac and Mrs. Cake can be assumed to be much more difficult to carry. To simplify the data set, considering there isn’t a solid mathematical formula to calculate carrying abilities, let’s assume that on average:
•	A pony weighing up to 300 pounds can be carried by an average, fit Pegasus within the normal weight range with noticeable, significant impact on flight speed and agility, but not to a severe extent.
•	A pony weighing 300-350 pounds can be carried by an average, fit Pegasus within the normal weight range with significant, severe impact on flight speed and agility, as well as stamina. Flights would be short and exhausting.
The theoretical load limit would thus be close to, but above current weight + 350 pounds. For the sake of ease, let’s say current weight+400 pounds is the absolute maximum for flight ability, as that leaves room for a pony whose normal BMI weight is 350 pounds to be overweight and factor into carrying capacity. This is mostly to avoid an edge case where a normal, fit, tall earth pony that happens to weigh 350 pounds won’t suddenly be impossible to carry if he ate a meal before the Pegasus lifted him up.
This isn’t an in-depth study of real world physics (knowledge I do not have) or what the different physics of Equestria would be, so we’ll be working with convenient assumptions and relatively fudged numbers.
By extension, the mechanics of flight for a pony weighing a certain amount without carrying anything can be assumed to follow similar logic. The maximum flight weight would thus be between 525 and 625 pounds. However, the difference in this regard is the added weight is directly on the body, comprised (in this example) of fat, and the pony is no longer fit and athletic. As such, the sliding scale of flight capabilities is pressed closer to the mobility-challenged side. To expand further:
•	Pegasi up to 100 pounds overweight would be similarly burdened to fit Pegasi carrying light ponies, but with a lower maximum flight height and stamina. A flight from Cloudsdale to Ponyville would be mildly tiring.
•	Pegasi 100-200 pounds overweight would be similarly burdened to fit Pegasi carrying heavy ponies, but with a lower maximum flight height and stamina. A flight from Cloudsdale to Ponyville would be exhausting.
•	Pegasi 200-300 pounds overweight would be similarly burdened to fit Pegasi carrying 350+ pound ponies, but with a lower maximum flight height and stamina. A flight from Cloudsdale to Ponyville would be almost impossible without breaks.
•	Pegasi 300-400 pounds overweight can fly, at most, a couple feet off the ground and only for a short period of time. A flight from Town Hall to Sugarcube Corner in Ponyville would be like a marathon. A flight in the opposite direction might be impossible if the Pegasus happened to acquire some goods from that bakery.
•	Pegasi 400+ pounds overweight are permanently grounded. Their innate physical strength pales in comparison to Earth Ponies, but is moderately higher than Unicorns. 
Pegasi that are heavy enough to be either incapable of, or barely capable of, flight are already at a very high level of obesity, with bellies hanging close to the ground. Similar to Earth Ponies, mobility is maintained even when the belly reaches the ground and spreads outward between the legs on all sides, but speed of movement and stamina are much lower. Total immobility, depending on the Pegasus’s strength, can sometimes be reached while the pony’s hooves are still able to reach the floor. In total immobility, some device or assistant is required for acquisition of food and other bodily needs. Wings, useless for their primary task, are still helpful for other needs.

Unicorn
175-275 pounds
A Unicorn is simultaneously at the greatest advantage, and at the greatest disadvantage in this field. Magic can counteract any effects of weight gain, to certain extents, but Unicorns are physically the weakest of the three species. They don’t have the raw strength of working Earth Ponies nor the athleticism and lean muscle needed for flight that Pegasi have. On raw strength alone, a Unicorn’s stamina, mobility, and carrying capacity would be lower than all others, though I would presume it’s as rare as it is for Pegasi for total immobility to be reached while hooves are still capable of touching the ground.
According to the suppositions I’ve laid forth so far, it can be assumed that Unicorns would have the lowest metabolism of all the subspecies, and as such would gain weight the most rapidly and/or easily. An Earth Pony or Pegasus could easily be a lazy glutton and fatten up quickly, but their inherent subspecies metabolism designed for physical labor and flight, respectively, would have a measurable impact. Conversely, a Unicorn is not an athletic nor laborious subspecies, and even in the theoretical case, mostly thought up in certain fetish circles, that mana pools are fueled by fat, the efficiency of magic use is such that regular spellcasting wouldn’t burn a major amount of calories the same way labor or exercise would.
In addition to Unicorn life being more leisurely on average – at least, based on known information about Canterlot – nearly all actions that would otherwise require physical labor can instead be performed using magic, requiring no physical effort on the Unicorn’s behalf. As such, another duality is presented: Unicorns will both feel the effects of their weight gain earlier and later than other subspecies. A Unicorn could, theoretically, laze in one spot all day long using magic for interactions and having eating be the only physical effort they endure. Excess weight has a greater effect on a unicorn’s weaker physical strength, but this is only noticed by the relatively sedentary population on the occasion they may have to walk a long distance or a busy day at work. 
A Unicorn’s magic is its strongest feature in any form, and that is no less true in the land of obesity. A Pegasus or Earth Pony will always have to get up to obtain more food and drink, or have a friend or servant help. A Unicorn can, whether he’s just started to sprout a potbelly or hasn’t been able to touch more than his belly to the floor in a decade, simply use magic to obtain more food, clean himself, scratch an itch, or invite guests in. While mobility is still available, a Unicorn can cast a strength enhancing spell to make walking around town an only mildly exhausting effort, even if a lifetime of excess and indolence has left her with a belly she can hardly keep from touching the ground. 
As a whole, while Unicorns, on a raw strength scale, will fall behind Earth Ponies and even Pegasi in physical capabilities, their magic use gives them an edge above the rest.

Hybrids
My theory for subspecies hybrids in Equestria doesn’t quite follow any known, realistic forms of genetics. However, this is where I will draw the “fantasy world with many unknown differences from our own” card, and point out that by all known laws of aviation, Pegasi would not be able to fly using Earth physics. Also because I say so.
In effect, a hybrid would have the same benefits and weaknesses between a regular pony and a fat pony. Following a Dominant-Recessive naming format: A Unicorn-Earth Pony hybrid would be thicker and stronger, while weaker in magic. Likewise, a Pegasus-Earth Pony hybrid would be thicker and stronger, while weaker in flight. An Earth Pony-Unicorn hybrid wouldn’t be horned nor magic-capable, but would be a thinner and weaker Earth Pony. An Earth Pony-Pegasus hybrid would also be thinner and weaker, without wings nor flight, but would be more athletic. (In theory, this hybrid type could sometimes birth a Pegasus with dysfunctional wings, but I like to think Scootaloo suffered a crippling injury while she was a foal, which terminated her parents and gave her a lifelong fear of being trapped in a cave.)
A Pegasus-Unicorn hybrid would not, of course, create an Alicorn, but it would create a Pegasus that’s somewhat frail and weak. A Unicorn-Pegasus hybrid would be more athletic and physically able, but weaker with magic.
In a way, the hybrid system works as such: The Dominant gene in a hybrid affects their apparent subspecies, while the Recessive gene in a hybrid acts as a modifier for physical properties.
For handling obesity, the type of hybrid can heighten or severely dampen the pony’s abilities. From strongest to weakest:
•	Unicorn-Earth Pony: The enhanced physical strength makes up for the weakened magic, while the availability of magic still helps make things easier even into the immobility stages. Arguably, this is the strongest even compared to purebreds. However, if you were to pit a 600 pound Earth Pony against a 600 pound hybrid of this type, the Earth Pony would still prove stronger and longer-lasting.
o	Average weight: 212.5-312.5 pounds
•	Earth Pony-Pegasus: The strength loss from the Pegasus mixture isn’t as severe as it would be with a Unicorn, which slates this mixture slightly above the rest. A hybrid of this type isn’t much weaker than the Unicorn-Earth Pony mixture.
o	Average weight: 187.5-287.5 pounds
•	Earth Pony-Unicorn: The strength loss is significant, but slightly edges out the next hybrid type in line.
o	Average weight: 212.5-312.5 pounds
•	Pegasus-Earth Pony: While this mixture is technically stronger than Earth Pony-Unicorn, the extra bulk of the Earth Pony genes means the Pegasus’s flight abilities are significantly hampered. Hybrids of this type are often teased as being fat, even without actually being overweight.  A higher base weight and weaker wing strength leaves these hybrids unable to fly at a weight range of 400-430 pounds.
o	Average weight: 187.5-287.5 pounds
•	Unicorn-Pegasus: While weakened in the magic realm, the Pegasus mixture provides enough of a strength and athleticism boost to edge it above the next hybrid type.
o	Average weight: 150-250 pounds
•	Pegasus-Unicorn: While only a little heavier than a regular Pegasus, the Unicorn mixture weakens the athleticism and stamina, leaving the hybrid worse at flying and overall weaker.
o	Average weight: 150-250 pounds

Bat Ponies
150-250 pounds
For all intents and purposes, I would consider the Bat Ponies identical to Pegasi in terms of ability to handle obesity. I designate a slightly higher base weight range to cover the wing composition differences and the fact Bat Ponies, in their rare few canon depictions, look a little more muscular, primarily seen as soldiers and warriors. Based on the available information, this is the most accurate depiction I can grant.