You have to bear in mind that every eating disorder is a very personal thing a varies greatly from person to person, but as a girl who suffered from anorexia starting at the age of 13, I can tell you that for me personally, weight had very very little to do with it at the beginning. If you read in to eating disorders, you find a common trend: Anorexics generally come from controlling families or lifestyles, bulimics generally come from chaotic families or lifestyles.
This proved to be very true for me. I come from a very strict, controlling, and over-achieving family. I felt as though I had absolutely no hold on my life whatsoever. So what did I turn to? Food! Naturally! Like a smart, healthy person. For me, my diet was a form of self-control and defiance. It was only later that it turned in to a body-image obsession. I was basically a textbook anorexic.
My very close friend suffered from bulimia. She was a textbook bulimic. Divorced parents who fought constantly, chaos, etc. This was reflected in her respective eating disorder.
As you can see (at least in our cases) our eating disorders were sparked by our environments and emotional turmoil. The media itself had rather little to do with it. I feel like media affects body-image, but that alone is not enough to drive someone to a full-blown eating disorder. It is an accumulation of many factors.
I can only speak from my own personal experiences and express my own opinion.
There is an incredible book that I have recently came across called Gaining. It goes more in-depth with regards to what I mentioned along with genetic factors and much, much more. If you're truly curious about this I would most definitely give it a read! At least a skim-over.