I guess the difference is work load done during a game.. The second rowers in rugby U (as opposed to league) tend to be 6'5-7 weighing 130Kg.. so smaller than a line backer.. but then linebacker is an immovable object.. Dont ask him to run for 90 minutes..

Im not sure what the average super 14 player does but the Aussie rules guys (6'6 weighing 90kg) run 17+ km every game.