my breasts seem to have stopped growing

I started my period 2 yrs ago and my breast seem to have stopped growing, they look the same in my opinion. Their just to small to be fully grown. All the woman in my family happen I be big breasted and mine just seem to be to small so yes I am a little worried! And advice/help? I was born 12 weeks early so can my early birth have something to do with my development?? Can it slow down growing. And I am 15 and turn 16 in sep. Please any advice would be helpful! Please help :) thanks