Originally Posted by
Arashi_Leonhart
Probably a bit of both. Because of the way the top of the kimono bunches before tightening beneath the obi (belt) and that the breasts are kind of sitting atop the obi, any "size" is meant to be hidden in the way the fabric naturally settles. Unless you're, say, Sakura or Arc or someone, they're naturally going to be fairly concealing anyway. And historically the culture wanted women to be concealed of that kind of appearance for a sort of similar reason why Muslim women wear burqa.
Dunno about the art thing, though it wouldn't surprise me if "draw kimonos in traditional manner to emphasize straight-body look" is taught in art school or the like.