Demo, are you saying that typical linemen are usually faster than linebackers - or just that the only way a linebacker, being lighter, could get away with being a lineman would be by having extra speed? If the former, that's surprising - those in the line look as though they have huge strength, but not great mobility. I know strength helps sprinting, but the extra weight they carry wouldn't.
Yes, the largest NFL players are bigger than rugby players. Here's a
description of the current Welsh squad as an example - 6'6" and 250 pounds for the larger ones.
Good rugby players have to be more all-rounders than American footballers - 15 players in a team, but that's for offence and defence, which switches while the ball is in play. And while there are more stops in play than soccer, it's close to continuous play - which would, I think, put the 350lb guys at a disadavantage by the end of an 80 minute game. And it's not true that rugby doesn't have 'plays' - there are more 'set pieces' (ie starting from a known point, like a scrum or a lineout) than soccer has. Nowhere near as many as American football has, of course - but one of the things that looks not so 'sporting' from a European point of view is the coaching team having so much control over the game. Our tradition is much more that it's the players on the field who dictate the tactics - the coaches having coached them before the game. Having radios inside the helmets definitely looks like cheating.
The continuity of rugby is good too - while supporting someone in American Football is basically a matter of blocking, in rugby it means being in the right place to receive the pass, or take the ball in a ruck or maul, thus keeping the move going. The defenders have to constantly adjust to the attack, and consider how to switch into attack if they get posession.