Found what I think is an oversight. So, when you have a (non feeder, normal personality) partner and they move in with you (player WG on in this scenario, for context), then become immobile, you can normally usually only have your partner eat and gain with you, or choose to make only your partner gain, entirely depending on whether you choose to eat with her or not. As you know, there's no option to eat independently of them due to the fact that they take place of the carer and don't have the order food option, effectively meaning there's no way to make yourself the fat one in the relationship. HOWEVER...
If you get yourself addicted to Dizzy's before becoming immobile (either by opening the Dizzy's yourself or by having a feeder girl leveled up high enough to open it up before dumping her for the normal personality girl), you get the option to order Dizzy's delivery, which has ONLY the player be the one eating.
As a result, it's entirely possible to theoretically keep a normal personality girl mostly skinny whilst you, the player, blow up, depending on how much you have to feed her for the affection gains so she'll agree to move in.
Problem is, and this is why I think it's definitely an oversight, is that there's no dialogue for this situation, AND if you eat yourself into the care home, whilst your partner WILL continue being your carer (likely left over code from the feeder girls), all the girl's dialogue will still be the same as when you are living at home with her.
Although this clearly wasn't intended, I hope you lean into this oversight instead of removing it. It's essentially a happy accident that moves us closer to the possibility of being the "fat one" in a relationship with a non feeder girl instead of ALWAYS having to be the feeder or mutually gaining.