Anorexia is technically the fear of gaining weight/consuming food, yeah, but the thing is that it's mostly an irrational fear, which is why it's under the umbrella of anxiety disorders as well as eating disorders. Rarely will you find someone anorexic that is actually overweight, because they aren't so much trying to diet as they are trying their damnedest to AVOID gaining weight.
Also, there is a variety of diagnostic symptoms for anorexia, the physical ones for the most part closely mirroring the symptoms of malnutrition.
But anorexics see themselves as really fat. That was the point.
They never are, though.
It's an irrational fear.