Tbh I don't think it would change much. I think his issues with his body would still be there, as he idealizes an androgynous "doll like" aesthetic and even if he truly were more androgynously built I think he would still be striving for that. Body dysmorphia works in the sort of way where it doesn't quite go away when the goalpost is "reached", instead the goalpost is moved.

I don't think his body type would affect his personality either, though he still would have been probably bullied as a kid for being girly even if he did "look" girly.

On the flipside, if Shu had been afab I do think their behavior could have changed because the bullying for liking girly things would have been slightly different. I do think they could have still been bullied as a child (girl trying to play with the boys, being called stupid and made fun of for misogyny reasons). I think either way, cis or trans in this scenario, their performance would be similar to current day Valkyrie, reclaiming femininity as something that isn't something to be ashamed of and that can be powerful in its own way.

Usually when I change things about characters I like to transpose their backstories / narrative themes. I do tend to re-structure Shu's body issues with cis!girlShu by giving her curves, as a doll is a sexless perfect aesthetic representation of the ideal body. Being tall with curves could give her a complex of not feeling small/pure enough, especially with how bodies like that tend to be sexualized (something she would disapprove of).