It’s very common for your breasts to grow at different rates while they’re developing. Usually they’ll look about the same size by the time they’re done growing. If the size difference bothers you, you can try foam or gel inserts that fit into your bra or bathing suit. These inserts are sold at specialty bra and lingerie shops.
Sometimes breasts can still be really uneven (different by more than a cup size) after they’re done developing. If you’re unhappy about the difference in your breasts’ sizes and your breasts have finished growing (3-5 years after they’ve started developing), you may want to talk with your health care provider about the benefits and risks of cosmetic surgery.