I think these two characters clothes or bras are somehow looks thick(or like they're wearing pads) despite depicted as thin.
I don't know myself honestly, at least not other people.
The only chest I managed to play with WAS my sister's, they are quite compressed within clothes(and bras), then expanded quite a bit when bare.
(I won't disclose elder or younger. It was all in the past, not nowadays afterall, and will never intend to do more.)