I am a psych student and we are thought that a disorder has to negatively affect your or others functioning and health to be classified as such. You can have a chair fetish if you want to, as long as you are not bothering anyone else and you are able to function normally its not a disorder. The negative effects of a disorder has to be visible or reported for a diagnose. In case of anorexia, the requirements for its diagnosis is clear - you continuously refrain from food and put yourself in a dangerously low BMI. You can just look at DSM 5 (a book of disorders and how to diagnose them) and see for yourself. An obese person can't be anorexic because their BMI, although in a dangerous zone, is on the opposite polar. Now, refraining from food on its own can be detrimental to health, but all of us can guess that when these 'obese anorexics' say they starve themselves they just get diet coke with 10 pizzas instead of normal coke.  As a final note, I find it funny that anorexia is considered a disorder and obesity isn't.