As others have said, the question is rather vague. That being said though if you’re looking for ways to make larger models you have a few options.
2D: If you aren’t too amazing at drawing you could always commission someone to do your models for you or maybe reach out to an artist here that is looking for a project to go in on. Another option that requires a little drawing skill is you can take your assumed current character art and draw over them just a little bigger and save that, then repeat that process for your different stages. Won’t look too amazing since the pose never really changes but it’ll convey the point.
3D: A simple program to use for this is DAZ3D. It’s all sliders so you can ramp up to the weight you want, but you generally have to buy the morphs, although some people do put them out for free. A lot of folks use this program for their games since it is very easy to use. The main drawback to this one is once you hit a certain size (tbh it’d be nearing immobility stages) the morphs can look kinda bad. A great example of a game using daz art would be Tiggertoo’s “The Weighting Game” or SirMister’s “Gain of Life”. You could also look into editing current models in a program called blender or even creating your own, but given the question you asked and I mean no offense by this, these may be a little out of your scope so I’d look into the other items I posted.
Anyways, hope this helped and good luck on your game!