Hormonal birth control simulates pregnancy. That's the way it works. A pregnant woman can't get pregnant again, because ovulation is hormonally suppressed. So birth control pills drop pregnancy hormones in your body. And you body "believes" that it is already pregnant and doesn't prepare any new ovum for fertilization. In pregnancy, breasts grow, because they prepare for breast feeding the baby.
Also, birth control pills can increase your appetite. Because pregnant women need to eat more. And your body "believes" that you are pregnant. But as you are not, you gain weight. Heavy women usually have larger breasts than slender women.
unfortunantly this is not the case with everyone, not all girls expirince breast growth while taking birth control, and for the ones that do its because they gained weight, and weight gain will obviously put weight al over your body
Because combined oral contraceptives are designed to hormonally mimic pregnancy. During pregnancy, your breasts get larger.
Because birthcontrol makes you gain weight, and some of that weight might go to your chest.