Breasts are often uneven in size or shape, especially during puberty. Around the age of 20, the difference usually begins to even out.
Breasts are mainly made up of the glands that produce milk, which are joined to the nipple by tubes. The glands are surrounded by fatty tissue.
Your breasts get slightly larger and more sensitive for a few days every month; this happens because hormones are preparing your body for pregnancy. After your period, they go back to their usual size and shape.