Sorry to have to burst your bubble but the game never closely followed the laws of physics. Case in point, the MC's powers and how they defy the laws of conservation of mass, all the way from level 1.
The MC either always has the same mass but changes size(volume), but that would mean he would be constantly breaking though floors, maybe through the ground since he can become giant-sized, while being able to somehow increase his strength and maintain his posture. So it's not like he's just inflating himself with air. He is also able to turn into a fly and actually.. fly.. which makes no sense even with his regular human mass. Even him being able later on to fly while human-shaped, without hollowing his bones, basically lowering his mass, should be impossible; it would be more like jumping really high and then gliding. Well, superhuman strength and super fast flapping maybe fix the flying issue but the wings would have to be absolutely huge, much bigger than what we see in the game and the flying itself would always be really annoying and wonky. Anyway, I am pretty sure it was mentioned explicitly that the MC is changing his mass/weight at these points though, so it's out of the question.
Or the only thing that would actually sorta make sense would the typical comic book/anime cop-out: pocket/alternative dimensions where the MC stores his extra mass. So a meat dimension, ain't that something! Or he is converting monster power, which could be considered a form of energy I guess, into mass/flesh and back into energy at will, in a manner of seconds. Yeah that's probably it, not completely unscientific I suppose.
When it comes to superhero shenanigans this pretty much comes with the territory, doesn't necessarily mean it's faulty writing. It makes sense with the whole sorta but not really Lovecraftian setting also.
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