The game is simply amazing, we can see that you put a lot of effort into creating it, the animations are very fluid, and the sounds fit perfectly, but there are some things that would be nice to add, I'll leave here my suggestions:
More belly sizes: I know that creating all these sprites is very difficult, but I think it would be interesting to add the belly size 0.5, what does that mean? Well, currently we can eat up to 3 prey, but when we manually digest the prey, there is an extra belly size, we get that size if we don't digest the last part which is when it spits out the bones, when we eat another prey that size is "ignored", my idea is that this size is kept in the next belly sizes, so we would have size 0.5, 1.5, 2.5, 3.5 and possibly a 4.5, because you said you want to add size 4 and make it still move in some way.
So my idea is that size 4.5 is the size in which she actually loses mobility, being able to move again only after digesting the extra size.
New Enemies:
I have 3 new enemy ideas:
Enemy with big breasts: when we eat this enemy there is a different animation, where the breasts get stuck in the mouth, and then they get swallowed, the size of the breasts makes this enemy occupy a space of 1.5 prey.
"kamikaze": this enemy works like a kamikaze, but instead of blowing herself up, she runs towards Mia and enters her mouth, I just don't know what would happen if this enemy tried to enter Mia with a full belly, it could give a normal attack, try to enter and be regurgited after that or make Mia regurgitate the prey swallowed earlier and enter.
"kamikaze" duplicator: this enemy works exactly like the previous one, but after entering it creates a copy of itself, occupying a space of two prey, if the copy exceeds the limit of the belly it is regurgited after been created.
Weight gain: it would be interesting to add different forms of weight gain, such as an increase in the entire body, or in specific areas, such as the breasts or butt, I would love to see Mia with big bouncing boobs.
Test mode: a mode to test skills would be interesting, a flat place with the upgrade machine and a machine to summon enemies, in this mode you have infinite DNA, so you can make all the upgrades you want and test the skills.
I'm going to make some fan art and post the link to my deviantart, if you like it feel free to use it, I would really like to see this game finished one day.