Rugby players are a joke compared to the massive size, speed and strength of NFL players, even a lot of the college players are much bigger and stronger than rugby players. "B-b-b-but rugby players can run longer!!!111" lol.
NFL players don't hold back when they hit each other, they launch themselves at full force thanks to the pads.
Good luck taking a hit from a 6'7 290lb defensive end like Julius Peppers with blazing speed and amazing strength. And no he's not fat, he's got just 6% body fat and is considered a finesse player.