The point is that the characteristics of the players for certain positions are similar, the physical characteristics.
I think this shows a fundamental misunderstanding of what "physical characteristics" means.
Yes, there are players who are around the same size. However, American Football is a completely alactic sport. There is little, if any, requirement for lower threshold energy systems. Training reflects that and performance reflects training. The athletes are more explosive, generally stronger and certainly faster. You say not all tight-ends run 4.77? Most starters usually do at NFL and D1 collegiate levels. I, myself, ran an electronically timed 4.73 at 117kg in a tryout for Orlando Rage in the XFL. Guess what? Too slow for a Defensive End or Tight End. And that was 14 years ago.
Conversely, rather than being a peak output sport, Rugby is a capacity sport. Players at the same weight and height will not be as fast, strong or powerful as a commensurate American Football player. They will, however, be able to retain that capacity longer and recover faster from more intense work - even while still working. Rugby players need to develop their lactic pathways as that is where the game is played. They also need excellent aerobic capacity to improve their recovery.
Further to that, you say that the tackles are too obese for modern Rugby. That would imply that players other than tackles would manage. They wouldn't - and don't. Take a look at what is going on the the NRFL at the moment as they try to make Rugby athletes out of wannabe NFL players.
And as far as tackles being obese, that is an insulting absurdity. Most run under the American (or Australian) average bodyfat. And in the case of Offensive Tackles, run backwards faster than most human beings can run forwards. Obese is not the right word.
Woah, I didn't think all of that was going to pour out when I started the post.