David decided to take Mariah's advice and have a big bowl of ice cream, even though it was first thing in the morning. He realized he must've been starving this body and that's why he was so grumpy. He felt much better as the sweet treat filled him up.
He realized Mariah spent her whole life eating junk food and fattening herself up, and the best thing for his emotional health would be to stop resisting it and give into his new body's impulses. After all, he'd be living in here for the rest of his life. No point making himself miserable by fighting it.
Now that his belly was nice and full, he decided it was time to confront his new body. He looked in the full-length mirror once again, resolving to see himself as Mariah rather than David. He had something of a pretty face, almost, but it was more chubby than anything. He took a breath out of nervousness and, then, pulled off his shirt. He apparently didn't wear a bra the night before, as his breasts spilled out and wobbled everywhere before settling down on top of his belly. Well, there they were. It was gross to think of looking at his sister's boobs, but thinking of them as his own instead, it was kind of exciting. He lifted them, dropped them, prodded them, felt the strange textures of fat and ducts inside them. "Ok," he thought to himself, "these are my boobs. I can do anything I want, and I don't need to ask anyone's permission. They're all mine!"
He squished his belly around, feeling the fat that spilled over his waistband, his muffin top, the back fat. He jumped up and down a little to see everything jiggle. It was somewhat embarrassing, but as he started to accept it, it actually wasn't that bad. It was soft, warm, fun to play with. And he really wouldn't need to worry about being in shape or not - he could just accept being a fat girl and let that be his identity. He could eat and drink anything he wanted and not have to worry about losing his hot teen-boy figure, since he didn't have one and it would be impossible to ever have one again. "This really isn't that bad," he muttered to himself in Mariah's female voice grinning with Mariah's face at her reflection. "Ok, Mariah, you've got this. You're a fat girl with big boobs and a fat ass. There. Now let's keep having fun with this body..." He looked down at his reflection's lower half, with pajama pants covering her female crotch... It was time to see what else this body had in store for him. After all, it was his now, and would be for the rest of his life, he thought, smiling.