[EllaC]

You know a friend of mine in her early 20's was very skinny like I am now and she had VERY small boobs. As the years went by she put on more weight and now she is possibly considered a medium sized clothing her boobs are reasonably big (consistant) with her new weight. So I think that yes the more weight you gain the bigger your boobs will get, that said for the most of us it will go to the lower regions first and breasts last. But based on what you are saying gaining a little more might be good for breast growth in your case. Also because of your age its a good time to eat good healthy food coupled with massaging to take the nutrients up to your boobs.
My boobs started 16ish I suppose but I still grew up into my 20s so there's still a chance you are growing slowly.
I don't think anyone can tell you what your potential for growth in cup sizes is as we are all different. But If I was you now with all the growth hormones and youth on your side Id be eating good food and massaging big time and I think I would have gotten bigger too with the right food