I think you're misunderstanding him a bit.
There are people, like myself, who grew up to 6'1/150lbs at 18 and have the frame of someone who could conceivably hold 180lbs comfortably, but not much else. There are people like Matthew Knies who was 200lbs when he was drafted, and can comfortably put 40 more -- his frame is absolutely massive.
Some kids have the ability to stack on muscle and weight from doing very little, while other kids put in the work, but can't seem to keep the weight on. It's not an issue of work ethic, but genetics.