The problem is that very few people have the genetic potential to be the size and shape idealized in our culture. That is the stone-cold, harsh reality of genetically mediated weight and shape regulation. This is because height is largely genetically mediated, meaning there is not much I can do about it. Many men, athletes etc. have as much difficulty trying to increase their body mass above what is genetically prescribed for them as people who are trying to decrease their body mass. These indivduals will often have to resort to health-threatening products such as steroids to get bigger than what their genetic code wants them to be.
Many people have weight difficulties as a result of disordered eating and lack of exercise.
The only way we can establish a genetically appropriate size and shape for someone is by normalizing food intake and exercise over a prolonged period of time. The road to recovery for an eating disorder is actually quite simple. One needs to eat reasonably, exercise reasonably, and make peace with your size and shape given reasonable eating and reasonable exercise. Reasonable eating is not hard to define. Any competent dietician can recommend a meal plan that meets normal nutritional needs, guards against the person entering into a semi-starved state, and is a meal plan that the individual can follow the rest of her life.