A rugby union player has to run around the pitch for 2 bouts of 40 minutes [about 40 minutes of 'ball in play' time]. According to this article - https://qz.com/150577/an-average-nfl-game-more-than-100-comm... - the average NFL match is 11 minutes of action, in [average] 4s bursts, over a 3+ hour period. Many NFL positions can optimise for muscle mass without worrying about stamina and sustained speed.
FWIW the measured tackle and at least one footage tackle [8mins 26s] in your linked video - https://youtu.be/TSc_Gxq1two - look illegal to me, lifting a player and pushing them down with head/shoulders first towards the ground (spearing, https://en.wikipedia.org/wiki/Spear_tackle) is a red-card offence.
Not just mass. What I often hear NFL athletes and trainers talk about maximizing "explosion" in their movements.
https://www.youtube.com/watch?v=W7tGY-VDx3o
Don't have much commentary on the subject and just wanted to share the vid. I'd only think that a difference like that can't really be resolved to padding.