According to the December 1999 issue of the journal Eating and Weight Disorders, their fears probably are unfounded.
When the Western Psychiatric Institute and Clinic of the University of Pittsburgh Medical Center treated 94 women for eating disorders, they found that successful treatment generally didn't lead to excessive weight gain. Most of the women were able to deal with their eating problem and their weight concerns at the same time.
But there's a caveat. Younger women were more likely to keep their weight down after therapy than older women