Every woman has some asymmetry in her breasts and that is perfectly normal. For most women, the difference in size is barely noticeable. Some, however, have a visible difference, which may be problematic in terms of cup size.
As long as the difference in size doesn't cause any health issues, you can use some simple tricks and choose the right bra to visually even things out. Ideal for an asymmetrical bust are bras with removable cups, T-shirt bras, push-up bras and bras with moulded cups.