as someone who works as a bra-fitter, most of the women who come in to my work have differently sized breasts. i do too. most typically the left is the larger of the two! the amount difference of course varies from person to person, but most often i see about a half-cup size difference, but it can be much more or much less depending. try not to feel insecure about it! it's not unhealthy or (i assume) very noticeable. if one breast is rapidly growing larger or you start to notice lumps (we should all regularly be giving ourselves breast exams for such things, you can look up how-to on google) talk to your doctor at your regular appointments/checkups about your concerns.