Is it possible for size to be reduced by athletics?

I am 12 and I have been doing gymnastics since I was 8 years old. currently I go 12 hours a week. all my friends have bigger than I do so I am a little worried that my will be reduced because of my gymnastics. yesterday I saw an adult who was completely flat. she looked about 35 to 40. (i was at the pool) that made me even more worried. please help.