It is not unusual for women to have one breast smaller than the other: Breast asymmetry may occur in half of all women, possibly because of varying amounts of female hormones, including estrogen and progesterone, that affect the size and development of breasts. You can increase the smaller one with resistance training and other exercise to build the layers of muscles of the chest wall, pectoralis major and minor, that support breast fatty tissue. The result: a muscular lift for the smaller side.
Do chest presses with a single dumbbell that is very light, lying on a flat bench. Use a 2- to 5-pound dumbbell and lie on your back on the bench, setting your feet flat on the bench, knees bent. Draw the lower abs in toward the spine to protect the lower back throughout the exercise.
If the left breast is smaller, hold the dumbbell in the left hand. Place your right palm on the center of the sternum. Bring your left arm to the left side, bending it at the elbow so the elbow joint forms a 90-degree angle. Keeping your left wrist straight and dumbbell parallel to the floor, extend the arm to almost straight before flexing the elbow and lowering the arm down. Do three sets of eight to 12 reps. This works pectoralis major; you should feel them contracting with your right hand as you do this single-arm dumbbell chest press.
Exercise the smaller supporting chest muscle, pectoralis minor, in one-arm dumbbell flys. Remain lying flat on the bench, feet flat on the bench, lower abs engaged.
Place your right palm on the far left side of the collarbones near the left armpit to feel the pec minor work in this exercise. Holding a very light dumbbell in the left hand, start with the elbow bent and dumbbell perpendicular to the floor, raised above the center of your chest. Lower the arm to the side, pausing for a moment before lifting the dumbbell back to the top. Do three sets of eight to 12 reps.
Use a foam roller to do exercises that will stretch and exercise the entire chest wall for symmetry.
Select a foam roller that is 4 to 5 feet long so that when you lie back onto it, your head and entire spine are fully supported on it. Set the feet flat, knees bent, abs engaged as you did on the flat bench.
Place your palms together over the center of the chest and sweep them both overhead near the floor above you for 10 reps.
Next, bring your hands above your head, elbows bent and keeping the arms bent, draw your elbows down to your sides, then return to the starting position, for 10 reps.
Finish by crossing your wrists in front of the chest, elbows bent and keeping the elbows flexed, and draw the elbows toward the floor. Do 10 more reps.
These stretches lengthen and strengthen the muscles that support the chest, pec major and minor. It is important to do some exercises that provide uniform strengthening for the entire chest to not create muscle imbalances that can lead to injuries.