Stop trying to rationalize it's realism. It makes you look slow. Just point out the obvious. The game isn't realistic and it takes place in a world where ALL KINDS OF MAGIC exists, including the character in question. With magic, nothing can be rationalized. Since it is unexplained, it is left up to the reader's imagination as to why he got buff. Any reason is a viable reason when it is left unexplained. It is good writing to not give every single detail.
Like Star Trek or some other nerdy show for example. When they are flying around in their space ships and they are giving each other orders while flipping random switches all over the place.... Does anyone really question how unrealistic and fake it is? Nope. They suspend their disbelief and believe that those switches are actually serving a meaningful purpose.
The difference between this and the nerdy space crap is that people found something to be outraged about. Don't give in to the outrage. Just let them outrage. I understand that a lot of people feel the need to outrage because this is a double standard for all of the feminists who always cry about female characters' depictions. Both sides of this argument are now outraging irrationally over something stupid and meaningless.
I understand that one side of that argument is being censored while the other side is not.... That is definitely enough merit to earn some outrage. But people need to take their focus off of a single damn video game character and actually talk about the primary issue rather than a tiny symptom of the problem.