I always assumed stretch marks on breasts were something everyone had! I mean, when you think about it, in order to grow them to begin with the skin has to stretch. I've had marks on them since I can remember. Embrace it!
I'm a C cup, always have been. I've lost about 50 pounds, and haven't noticed any change in size, just a smaller band size in my bras. Same with when I gained weight...breasts stayed the same size all through college until my highest weight a couple years ago. A little more of the "deflated" look after losing but I think that's because I'm in my late 20s, not because of weight loss! At 19, I don't think you have much to worry about in that area
I definitely will be trying to work on my chest muscles when I'm closer to goal weight to see if that makes a difference in perkiness.