I'm worried about cancer

EVERYONE has a boob that's bigger than the other (well every girl anyway). If it's extremely noticeable it could just be a cosmetic issue but it doesn't mean cancer. You can get it fixed later on in life through surgery if you really want to.

If either of your boobs feels lumpy or has a lump then you should definitely see a doctor.

In the meantime, why not talk to your mom or a close female adult about it? You may find out it runs in your family.