It’s not an old notion, it’s actually a newer notion relative to the history of hockey. You won’t find much data, if any, because teams don’t generally put kids in the NHL who are physically not ready, just to watch them get plastered. It’s not an issue in the NHL because teams make sure these kids are physically ready before they get called up.
There are only 5 active players in the NHL who are at least Mazur’s height and under 175 lbs. So he’s in the window, but just barely. He will benefit from more weight. He knows it and he’s working on it.