Some people like being fat. Some people like being women.

A person who likes bobs and vagene entering a committed relationship with a trans man would be toxic. Ditto for a person who likes fat people entering a committed relationship with someone who is trying to lose weight. That said, firstly, I don’t see anything wrong with a one-night stand for either of those scenarios, which is what the original question was, and secondly, more importantly, you’d have to stretch really hard to say that a person who’s into a little chub and a person who’s perfectly happy having a little chub entering a committed relationship is in any way problematic.