I'm 5 ft 3 and a size 10 in pants?

I'm a pretty good sprinter (6 mph is my comfortable pace, and 10 mph is my sprint). I'm 15 yrs old, 5 ft 3 (petite), a size S or XS in shirts, but a size TEN in pants! How can I get my thighs to be smaller?!!!!!!

p.s. The pic is of me-what do you think? Fat or not? Don't be afraid to be truthful!