To reduce the size of your breasts by exercise, you run for at least 30 minutes a day. It doesn't have to be fast pace or anything, but you need to be active to get your heart going. You will see results if you stick to your routine and have a healthy balanced diet. Try to stay away from fatty, greasy, and oily foods. Sugars too. Low in carbs, and high in protein. Protein will give you the fullness for your body, so you will not have snacks all the time.