Maybe Kei just THINKS they are much stronger as male and got an emotional powerup instead of an actual physical one. As in they could have done the same in the female body but just juvenile stupidity held them back.
Still dumb but it makes sense for the character I guess.
That is the most gracious I can be, because it's the same sort of logic that allows mothers to lift cars off their children and whatnot. Physical capacity goes out the window the moment there's an extreme-enough need for it, and then it doesn't matter who you are, your body will suspend the laws of physics to make it happen.
It's not "that Uesugi's a girl therefore Uesugi is weak", it's "Uesugi
thinks girls are weak, therefore Girl Uesugi is weak". And while I still think the stereotype is silly and wholly unnecessary as an abstraction, it would at least be consistent with how turned-around Uesugi is with the whole gender fiasco they've been forced into.