Actually in many ways eating disorders are not just about  fitting a preconceived standard set by others.  Often it is also an attempt at control with many young girls who suffer from it trying to find a way to control some aspect of their lives.  They often have overly high standards for themselves or live in a situation that places said standards on them or that removes control from their hands.  Not eating - or controlling what they allow their body to digest is control - it isn't about fitting in as much as people think it is.

That isn't to say that self image and conforming doesn't kick start it for them but rather that they begin to realise that they have control there and that is where it goes out of control.  Being told one is too fat or not good enough is a form of emotional abuse and abuse leads to a need to have some semblance of control.  As women/girls are pushed more and more into being "perfect" women - and I don't mean their looks but having a top career (or top grades for a scholarship) and being a perfect mother and perfect mate while always trying to please everyone the need for control continues and becomes stronger in our youth. 

It isn't really that culturally they feel they must be thin but rather that they feel that according to society they are lacking in some ways and it generally isn't their weight that is the underlying problem.

Many individuals with eating disorders can get to the bottom 2 rows of BH's images and they think they look too fat - they don't see the skin and bones that we do because it's not really about the weight to them but about their own self loathing and self image that may have nothing to do with weight.  If one focuses entirely on their weight to that amount of obsession they do not have to focus on any other aspects of their lives.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo