When I was 13 I was a 12AA now, I am 15 and wear a 14B. My friend she had an A cup, a month later she was in a C. Everyone is different, and we all grow at a different pace. My sister for example is 2 years younger than me, and we have the same bra size. Everyone is different.
It also depends on your genetics. If your mum is a small breasted woman, than chances are you will be the same as your mother, if she is a large breasted woman, you might end up with larger breasts. We all grow at a different time, dont fret, be happy you have smaller breasts.. Big ones can cause lots of different back problems razz
Source: General knowledge.