The breast tissue is made up of nipple, areola and mammary glands which are embedded in fat.
Fat is the major factor which gives the breast it's shape size and contour.
At 20 yrs, your weight is just 40....thus low weight could make you feel the difference in breasts.
But many a people have unequal face or breasts and even structure, so nothing is wrong in it. You can wear a modified padded bra to overcome the unevenness
Do chest exercises, to build up your pectoral muscles( on which the breast lies ) and have good nutritious diet to allow a weight gain of 5-6 Kg. this will add fats to your body and breast and even out them.
If this doesnt help, meet a plastic surgeon and get the opinion for doing any enhancement surgery