Only on that end!

"Only on this end, don't worry," you reassure her. After licking your lips, you can see she gets the message. "I have a lot to go, so I don't want you holding back," you remind her.
"You don't want to work for it? Maybe I want you to," she says, pouting.
"Listen," you say as you pull yourself off of her cock, making sure to keep your muscles tight to trap the cum inside. "You have gallons in there, I want it as fast as you can give it to me."
You swing yourself around to a 69 position. You make sure she gets a face full of your ass before you go down on her. Things go well on your end as she does her part on your rear end. You loosen up, and her cum flows into her mouth. She doesn't complain, or maybe she just can't. You just barely have enough time to take a breath each time you swallow.
You are dedicated to your task and so even when you start feeling full, you keep going. Jill probably did not expect to be helping, and tries to speak into your butt with her mouth full of cum. You can't understand, of course, so she tries slapping your butt to get your attention, and then pulling on your cheeks. You know why when you feel her belly rising as it fills; it must be uncomfortable for her. As a real person, you should have a lower capacity than her, so when you can't take anymore, she should still be fine.
When you feel no more liquid coming from your butt, you stop your work on her cock and the cum slows to a stop. After crawling off of her, you see her sack has diminished in size significantly. What has grown, however, is her middle.
"Look at what you did," she shouts.
"I look like a had a big meal myself," you say as you pat your middle.
"Big meal? It looks like I ate a bowling ball," she groans. "I don't have all of your internals to distribute the mass."
"Well, speaking of big things, you look like you are shrunk enough to move around. They are only the size of beachballs now."
Jill finds she is able to drag her legs out from under her swollen balls. She even manages to stand and walk about with some effort.
"Aren't you going to finish?" she asks with a hand on her thrust hip and another gripping her hard dick and pointing it straight at you.
"You look rather sexy like that. Besides, anything that you squirt up my ass is going to end up there," you laugh as point to her belly.