The only really big guys are the prop forwards, like myself, right at the front of the scrums. And even some of them are only big through heavy duty weight training!
..then you've got someone like me stuck in the middle trying to win the ball at the scrum!

(ex hooker!)
..and as for American football.... lol
There was a load of American footballers came over years ago, got challenged to a game of rugby. They were well up for it until they got told they weren't allowed to wear their armour... lol