We say so because that's the most likely outcome for him, statistically speaking. We know it's not gonna happen tomorrow, though. He's still high on all the fame and attention he's got and above everything, denial. Being alone is also helping him to get distracted, because he's out of a house where everything they talk about is his genitals. But we know that's gonna be a point where he realises all things he can't do and that's when he's gonna crack, childish or not.
And yes. Sex is perhaps the most important thing he's gonna realise he's missing. Specially because the surgery was related to that. He was told to have a vagina and he has a hole he can't use. This isn't like some cripple or a blind person being born blind. People like this learn to live with it. This is a normal human being told they could enhance their eyes to make it more like feline eyes and after getting the procedure, realising you has a worse sight than you had before. That bad.
For a boy or a young man, he eats normally. But if you see other troons his age, they aren't this big. And they also take a lot of different set of pills and hormones. There are young men who are obese too, and that's because they don't burn all the food they eat, and we know Jazz eats more than he should, even for being a man. Young men or boys who are constantly hungry do so because their level of physical activity demands it. And not only men, athlete women also eat more than women who dont' exercise (I mean real sports, not gym faggotry). Jazz eats because he's depressed and anxious.