I'm skinny, but heavy?

I'm skinny, but I'm heavy and I play tennis and soccer. I'm 60 1/2 inches, but how can I be heavy? My friends say it's muscle, but I still don't know. What is going on?
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+1 y
I'm 113 lbs. (as embarrassing as it is)
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+1 y
I'm 13 and not really skinny-skinny, but flat-stomach/fit thin
I'm skinny, but heavy?
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