In response to your question frogs, I can only give my own experience as someone who has never had an eating disorder. I was shocked when I turned up at uni and found that of my 'gang' of six new (female) friends, i was the only person never to have had bulimia, anorexia or an exercise obsession.
I was certainly not the slimmest of them (size 12-14), but I had almost no negative body image issues, whilst they had huge ones, despite being an average size 10.
I thought a lot about why this was, and came up with the following:
** I had never been forced to eat anything as a child. Food had never been a battleground in my house. if I didn't want to eat a particular food, my mum just took it away and said 'oh you are not hungry'. End of story. Hence food was never an emotional issue.
** I had never known my mother to be on a diet, to comfort eat or to talk or obsess about her weight. She wasn't particularly slim but was happy with her size
I don't know whether these are related, or maybe I was just lucky, but food for thought maybe? ( sorry terrible pun )