Why is my kid so skinny?

My daughter could finish a whole medium cheese pizza by herself, (if I would let her). But she's still a stick thin string bean. What's up with that? The kid eats and eats, but never gains weight. She eats breakfast, lunch, dinner, snacks and more. But she is still stick thin. She has been skinny her whole life. My spouse says that she is just getting taller and that is true. She is taller than most kids her age. The kid is almost as tall as me. It's not a medical issue, we've had her tested for those. Anyway, my brother used to be the same way. He could eat ALL the time and never ever gain weight. He was so tall, like six foot or more. But, he was stick thin. When I was her age, I was really skinny. But, I didn't eat lots of food, cause we were poor. She does eat lots of food. I'm not too worried. Anyway, do your kids eat a lot and still not gain weight? Are they really tall for their age?