Assuming we can do anything with variables, I think the best thing to do would be to have a variable track every time you fill one the girl’s food meters. Once a certain threshold is passed, then change the set of character graphics to reflect weight gain.
It’s been a while since I’ve played Huniepop, but if I recall correctly the girls would occasionally quiz the player about their weight. Depending on what exactly is possible, this question might have to be removed or altered.