If you've passed puberty and are still gaining fat in your lower body, it's likely that you're eating more calories than you burn on a regular basis. The fat cells in your lower body swell to store the surplus of energy. Just 100 extra calories per day can cause you to put on more than 10 pounds a year. If you add a soda, an ounce of cheese, a latte or about 3/4 of an ounce of chocolate to the calories you already eat, you may gain weight.