Sounds like this could well be an eating disorder. The best way to check would be to see a psychiatrist, preferably one with a special interest in Eating Disorders, or a unit like that at Tara, for a proper check-up and specific advice based on what they find. All anorexics are obsessed about their weight / size / shape, and few people other than anorexics / people with eating disorders, obsess about such things.
Dietdoc is an expert dietician, and dieticians are not trained in the proper diagnosis of Eating Disorders, though their advice on planning a diet as part of a combined approach to treating such disorders, can be very helpful.