The difference is that anorexia makes a person better looking, up to a point. It is characterized as a symptom of vanity. To become anorexic also takes willpower that the majority lacks and the crabs in the bucket can't stand that. Sure, at some point the anorexia takes the wheel and drives the sufferer's life, but to get to that point does take a lot of self discipline. Both the overweight and the underweight have disordered eating, but nobody sane thinks that butter golems are expanding themselves to be attractive. Also there's the obvious fact that the obese are now the majority and mob rule does apply.