Well they start them young and they grow them big, and many are on the juice no doubt.
You have a very large gene pool, and rich availability on nutrition (just choose the right sources). The only (ex) A football player I know about is Bob Sapp and while he is strong as an ox he has the stamina of a crocodile on land. Huge bodies and endurance dont usually mix well. In fact, endurance training will produce catabolic hormones like cortisol that actually breaks down muscle mass.
Genetics will take you far. Just look at russian Alexander Karelin during his height. 6'4" (193cm) and 130kgs. As an olympic wrestler he was able to lift and throw 130kgs heavy opponents like they weighted 80kgs. A line of such guys would not be broken and there would be some stamina/endurance as well.
http://www.youtube.com/watch?v=SfDDXpjFtP0
You might have heard a couple weeks ago, there was a very good running back from (I think) University of Southern Cal who dropped the barbell on his neck while doing his regular workout bench pressing...285 lbs! His spotter actually sliped and dropped it. But he's just a "little running back", not a big lineman!
285lbs is about 130kgs. That is achievable for a lot of guys if they work strength and power. A football is definately a power sport as I know it.
There are a couple of A football teams here at the north pole as well, but the sport is not catching on. Regular football is huge..
