The problem with this, though, is that it puts an insane workload on the dev. Like, some girls already have near 20 weight stages and most if not all have around 10. That's a minimum, then, of ten sets of renders for every scene in the game unless the dev puts in a minimum "weight gain" requirement - i.e., you must grow to a certain level at least; but even that defeats the purpose of what you're asking. There are some people that will prefer the girls not gain at all, then some who prefer they only go up one or two stages, then those who prefer three, four, etc. etc.
Plus, content-wise, it wouldn't even make sense. All the story scenes revolve around the fact that the girl is bigger than they used to be. If they stop gaining, the story no longer tracks whatsoever, and the dev has to then come up with alternate storylines, which again, is a ton of work.
I think people just need to accept the game for what it is, rather than what they want it to be. It's a fat fetish/weight gain/feederism game that has some very hot sex scenes; the focus is and always will be on the gaining, whether that comes across as breast expansion, ass expansion, or just in the belly/hips/etc. I think it would be cool if the dev implemented perhaps a point in each girl's storyline where you can choose to stop growing like with Bonnie, but that's already a ton of work considering the number of girls. Asking to stop at any point doesn't seem feasible.