The boys rested for a while after their orgasm. After a while, Theodore asked Alvin a question.

Theodore: I want some more. You want some more too, Alvin?

Alvin: You bet! That felt awesome!

Theodore: Great! I want us to do something different now, though.

Theodore turned around and put his hand on a nearby wall. He was leaning slightly and he pushed his butt forward, giving Alvin a good view.

Theodore: Do you think my butt looks good?

Alvin: Oh, yeah! Definitely! So big and round! The fat did wonders for it. Can i uh... can i touch it?

Theodore: Of course you can. Please do!

Alvin put his on Theodore's big, fat buttocks. He was caressing them, groping them and feeling their weight. He squished them together and spread them. After letting go, both motions resulted in a nice jiggling motion. Alvin once gained a raging erection.

Alvin: Your butt is so big and soft. And i love how it jiggles.

Theodore then grabbed his ass-cheeks and spread his ass open.

Alvin: Woah! So that's what a butt-hole looks like. Neat!

Theodore: Alvin... I... I want you to put your pee-pee in that hole.

Alvin: In.. in your butt? You sure, Theodore?

Theodore: It's clean. I've done this before. Sorta.

Alvin: Really?

Theodore: Yeah. Put it in and i'll tell ya.

Theodore put two fingers in his mouth, wetting them with saliva, and inserted them into his ass. Alvin was gently stroking his penis. The sight of Theodore's large ass getting penetrated by his fingers aroused him even more. Precum started leaking out of his penis. He didn't say it, but he was very eager to have this new experience. His glans touched Theodore's enlarged sphincter. It was slightly bigger than the anus he was about to enter. He started pushing himself in. It was tight, but the contribution of juices from both of them reduced the difficulty of penetration. The way Theodore's ass squeezed his penis felt wonderful.

Theodore: Oh, yeah! Oh, you're so big. You like that, Alvin?

Alvin: Yeah! Oh... That feels incredible! I guess this must be sex.

Theodore: Sure is! Thrust in and out. Slowly for now.

Alvin: You said you done this before?

Theodore: Yeah. First just my fingers while rubbing my penis. Then my whole fist. Then some things we had around the house. Then i saved up and bought a toy. A dildo. Big one. Almost as big as your penis.

Alvin: This feel good to you too, huh?

Alvin was talking while breathing heavily. As he was thrusting, his testicles were touching Theodore's. He lightly grabbed his brother's balls and held them close to his own.

Theodore: Yeah. It's my first time with someone else. I thought i would do it with a girl first, but boys are great too. I like both.

Alvin: My thoughts exactly.

He started picking up the pace. Theodore's penis was twitching as he was being penetrated. Alvin put his right hand on it and started masturbating him. With his left hand, he started caressing and squeezing Theo's big belly. His hand was slick with Theo's precum. Alvin's own cock was also leaking large amounts of the fluid right into Theo's ass. After a few minutes, both boys were close to their climax. 

Alvin: Oh, Theo... It feels so good. Oh... I think i'm about to shoot that white stuff again.

Theodore: Do it. Squirt your white goo in my big butt. I'm about to blow too. You're gonna make me even bigger. Even fatter. I want it. Please.

His brother's words of want and lust drove him over the edge and he ejaculated. The amount of semen this time was far larger than before. His legs trembled and he used hard, short thrusts, as his cock pumped Theodore's ass full of cum. Theo's belly got bigger. Soon the rest of him grew. Alvin's was rubbing his cock much harder now. This, combined with the pressure inside him and his new growth, made him cum too. Each squirt was like a jet from a small fire-hose. It quickly coated much of the wall in front of him. He soon grew too big to stand with his trembling legs and slowly sat down. Alvin kneeled behind him, his cock now squirting the last, small spurts of cum into Theo.

While this happening, someone was watching them. A few moments after the boys were done, they decided to join in.