You don't have to have big boobs you could still be growing
There isn't a way to speed it up. You probably are just growing a little late.
I know, it totally sucks! I've had my period for 2 years and my breasts are the same size as they were before that which is NO BRA SIZE!!! Seriously, I'm 3 cup sizes negative, but my mum is an A so I'll probs end up small-busted too. You may still have a chance, though, if the females in your family's breasts are average or on the larger size then you should grow eventually.
Yes you have to give them time to grow because I don't have much of mi on so just wait and give your body time to develop them and it depends on your age so just wait and see what god blesses you with