Why my muscle become bigger because do swimming ?

I always do swimming since Im a little girl till now 16 years old. I do swimming once a week , sometimes twice a week but rare. . . Because of that I feel my muscle is bigger and bigger. . . its especially at shoulder, upper arm and lowwer arm(especially bicep tricep), chest and thigh. . . its burden me because Im a girl and still teenager . . .what should I do , so I still can swimming and still have a little body. how to get a little body and muscle. so my body looking good. (my muscle doesnt come to bigger). . . and get taller . . thanks