After some experimenting, I think I understand how the weight / hunger systems work and I'd like to give some feedback on it:
It seems what's happening is that the weight gain is directly related to the hidden hunger gauge, where you gain weight if the gauge is high and lose weight if its low. So if you really stuff your face, the hunger gauge goes very high and you gain weight.
The problem I found with this system is that the hunger gauge doesn't appear to have a cap or reset after sleeping, so if you eat a lot, it goes super high and depending on how much you ate, you can go days to weeks without getting hungry and gain weight every day even if you don't eat a single piece of food.
In one of my saves, without knowing any of this, I really stuffed Ayume up for about a week. Once she got over the 55kg mark, I decided to make her thin again, except the hunger gauge must have been so high at this point that she kept gaining weight every day for about 2 weeks despite not eating anything all.
So I feel this whole system can be quite confusing and unrealistic. I think what could be done to improve it is firstly making it reset after sleeping and making Ayume wake up hungry. This would fix the problem of stuffing your face one day and not getting hungry again for a whole week.
As for the weight gain, you could implement a cap on the hunger gauge and make it so all food you eat after capping the hunger out causes Ayume to gain weight at the end of the day. If you don't cap the gauge out then you lose weight. And I think it would be nice if we had a visible gauge for the hunger instead of it only showing up when she's hungry or starving.
Sorry for the ridiculously long post. I hope it was helpful.