All of my kids were fairly thin , my youngest is skinny even noe that he is grown. His first born is 13 weighs 185 pounds, is 6 ft tall and very big boned. People look at him very odd when they find out how young he is , but for his height and bone structure he wieghs the exact amount he should. They use the mens growth weight chart with him.
My grandson has a very large frame. Since he was a baby he has had growth spurts and we were all worried at the size he was. The doctors did do some tests to make sure he had nothing wrong. tests came out fine. They are not worried because of how large his bone structure and his heigth is they say he's just going to be a big man, not overweight , but big.