The strongest, most powerful, fastest, heaviest guys in the NFL will be faster, heavier, stronger... than those in rugby.
It's simply a matter of fitness/appropriate conditioning.
The cardio and endurance requirements of rugby players limits how big they can be, and thus their top end "measurables".
NFL players have minimal cardio/endurance requirements in comparison, therefore can pretty much get as big/fast/strong as they can.
You then factor in that the NFL is drawing it's athletes from a much bigger pool of players, and therefore a lot more athletic freaks, and those athletic extremes are bumped up further.
And that many positions are far more reliant on athletic ability so less players are ruled out by a lack of particular skills.
I would say that the general level of conditioning in professional rugby could/should be a lot better than it currently is.
You look at a lot of guys, and hear the numbers they put up in the gym or in sprints or whatever, and they really aren't very impressive a lot of the time.
There are some phenomenal athletes in rugby, but equally there are a lot that are far below their potential.
It's a cultural thing more than anything from what I can tell, the USA has a very well developed strength and conditioning culture/industry in comparison to the countries in which rugby is a major sport.
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