Okay sooo i've tried to read up on this and it seems like a pretty common question, but with varying answers.?

If you are actually overweight, then losing weight will decrease the bra size around, but not so much in the cup size. You lose weight all over the body, not just in one particular spot- so the amount of weight you lost in the themselves would be negligible at best. Genetics determines where fat is stored, but that is primarily in terms of how close to the surface of the skin. Women tend to put fat on the hips and thighs, and in the lower abdomen, primarily. Men tend to get it in the belly and chest. So while there is a genetic component, it's pretty much in terms of being male or female. If you need to lose the weight, just do it and worry about bra size later. If you do not need to lose weight, then don't, either. At 16 you are still growing an adult body, and what you see as fat may be building material. Your final figure has yet to be determined, and things can always rearrange themselves a bit. But likely you will always be a D cup, so you can relax on that part.