@Avenger88id I don't think the spirits care about their body type or boobs, I think that the humans care about them. The girl doesn't want to show off a body with no curves because she thinks it's not good enough to be seen.
Reminds me of spirits in Seirei Tsukai no Blade Dance. For some reason, their preferences are like that of a stereotypical otaku - they like young girls, and the princess can always justify having fetish clothes with "that's what spirits like".
@dharman - it all depends on the religion. A priestess of a religion from a warm seaside place like Greece would not be wearing too much.