I think this is because we associate attractive with thin. A video about body image interviewed model agents. It is astounding the standards that have been set as the standard for models. The must have long necks and legs. The description of the height and weight of average women shows us that the figures of models are not common.
Women feel they have to fit the mold of a model.
A woman who is average feels unattractive. People associate thin with beautiful. Body weight plays an important part in physical attraction. If you have a nice body, people will find you more attractive. Men drool over models in the Victoria Secrets catalogue and Sports Illustrated swimsuit edition.