I'm sure in some cases that is what is happening. But most of the ones I know about, the eating disorder is just what the person latched onto to try to deal with horrible pain and suffering. They chose ED, because of cultural influences, but it could just as easily have been cutting (and often is both) or alcoholism or smoking or some other form of outlet. ED is an outlet, and you have to look at what the person needs an outlet for.

Normal healthy people don't generally just decide to starve themselves, even if they are a bit heavy.