I can't say I'm not one for larger sizes, but somehow what is in the game right now feels pretty final, I think it would be tricky to add more cause in the end, they can't really speak and barely think anything else apart from eating and how hungry they feel. What else could be done by that point? Maybe once you reach 99 national rate you can simply get girls outside the facility to bigger sizes than down in the chambers. But the interactions would feel a little... void of their reaction.
Again, I'd love for them to get bigger, but the execution would need to keep it in line with the game, it's a dating sim after all, and if the girl can't even react to their own mind/body changing or react to you, I think it would be too far from the initial concept. So if there ever are bigger stages from the blob, they should add a way for them to at the very least interact with the player :D. (Please do someday, one of my wives weighs 15 tons already lol.)