Its come down to being materialistic. best way to get people to buy things is to tell them it won't make you look so fat! try telling that to a young self conscious girl who is nervous about how the dress she wears on the first day of school looks, and the boys who tease her about how it makes her look like a circus tent when she barely weighs over 110. (this is just a statement not personal to me in anyway.) let her think that she is fat and she will buy everything to make her fit the society norms to look pretty.
Girls will do just about anything to look "pretty" when it doesn't even have to be that way