i'm 25 and I just recently went up a whole cup size. I dont think boobs really stay the same size forever like we think...I think they fluctuate with weight. A friend of mine went from a D to a B after she became a vegetarian and lost 20 pounds. The reason I just went up a cup size is because my metabolism slowed down once I turned 25 and i gained a few pounds.
of course they are going to grow and shrink depending on your weight... beings they are just fat yeah...
Mine seemed to get a bit bigger a few months ago. I had been doing a lot of physical work and getting muscles. I think it was just that my pecs got bigger so my boobs seemed bigger.
my boobs got smaller when i hit my 20s, and birth control def. did not make them bigger. i think you stop "growing" in your early twenties, but fluctuations in weight and muscle mass can change your boobs.