Ah, I hadn't considered the leg muscles. Those do weigh a lot. But does a 13 year old girl develop very muscular legs?
I mean, "middle school track runner" and "high school track runner" could be very different things. But if we're thinking she must be a skinny-minnie because she's a track runner, then, yeah, that goes hand in hand with having more muscle. If she's just the fastest kid in her gym class of video game addicts, then it's all moot anyway
But at the root of it, I come from a family of tall skinny women ... and a tall woman can weigh 150lb and still be plenty skinny.