Helpp! boob shrinkage!!!?

heyy im a 13 year old girl 5'0 and 107 pounds. i have reallyyy big muscles cause im a level 9 gymnast and i work out about 20 hours a week. since im a gymnast having big boobs is a problem...well im only size 32a but still my boobs are kinda big. im going on a diet and im gonna try to get myself to about 90 pounds. if i do this will my boobs shrink(i really want to be flat chested!!!!!) so if i get to be 90 lbs willl i be flat chested?? and also what else makes you be flat?(besides surgery obvv im only 13) thankssss(: