What etiology is implied when bulimia nervosa (purging type) leads to weight gain?

Bing eating is key : Bulimia is a condition marked by an abnormal eating pattern, usually binge eating. Associated with this, probably as a means of weight control are purging , self induced vomiting

Calories abs rapidly: Most people with bulemia are normal wt. Depending on what is eaten and time before purge, many calories are absorbed rapidly digestion begins in the mouth.