She didn't like the way she looked and asked you for a diet. I am concerned for her and for the many other adolescents who have difficulty accepting their bodies. Too much emphasis is placed on physical perfection these days.
Muscle weighs more than fat tissue.
Each time you exercise and gain strength, endurance and flexibility, you increase your well-being and develop muscle. Busy kids are happier kids. Stop checking the scale and keep an activity log. It's more positive reinforcement. Please don't compare your body with that of others your age.