What you eat (or don't eat) is the simplest thing for YOU to control. No one can make you eat (truly) if you don't want to...and no one can really stop you from over eating either. So for a young girl seeking order in the chaos she lives in, the simplest solution is to control what goes into her. She is controlling her body, her size. I don't really think their is a rationalization about the size of others...it is all about controlling their bodies/lives to a state of perfection.
Yes, but I would argue that seeing a lot people who lack control over their lives, and especially seeing one very obvious way in which their lives are out of control, influences the development of these girls' belief that they need to exercise such tight control over themselves. They aren't at all sure how people get so out of control, or why they can't regain control, and take extreme measures to make sure it doesn't happen to themselves. It's an overreaction borne of insecurity, and clearly intensified by some individual factors.
But my main concern is the exaggeration of the incidence of serious eating disorders, and the promotion among high school and college students of the idea that virtually any successful attempt to exercise control over one's food intake and/or weight is a warning sign of an "eating disorder". The result is a widespread, socially enforced rejection of any self-control over eating and weight, and in college this is combined with the all-you-can-eat dining hall formats, with very unhealthy results. Way more women are dying of breast cancer, the incidence of which is tremendously increased by even a very small weight gain, than are ever going to experience any harm from anorexia and bulimia.
When a college student gains 10 pounds (which is enough to significantly increase her risk of breast cancer), neither her peers nor school health officials will bat an eyelash. But when the same student subsequently gets serious about losing that 10 pounds and maybe even another 5-10 that she'd really do well to lose, peers start muttering about "eating disorders" and often going to hall advisers (who've also been brainwashed on this issue) to express grave concern. I've been on the receiving end of two of these ludicrous reports in the past few years, as an adult alumna who is in regular though unofficial contact with just a dozen or so students at any given time. But in over 20 years' association with this college, I've never once heard, first-hand or second-hand, a suggestion that someone who is gaining a substantial amount of weight ought to be referred to professional help.