P-h-o-t-o-s-h-o-p ^_______^ Too lazy to find an article but it was proven that she was self-conscious about her weight and asked to be made thinner in magazine photoshoots and whatnot. If you saw the Korean Special she did with BoA, it was pretty obvious she's not as slender as she'd like to be, her body type doesn't allow for it.
~ caps from the korean special:
Early on she sucked it in a lot, t'was amusing. Yes I know she probably has gone on numerous diets to try and get slimmer but she can't be "that" thin. Her torso has a cylindrical shape anyways, she doesn't have a pinched waist for an hourglass figure. Not really fat but she's certainly not thin.
(Also if you saw the interviews, even more cannon fodder for how shallow and completely clueless she is)