Nevertheless, the props of modern rugby aren't as fat as you think, the true fats are the Defensive Tackles of NFL. They weigh on average 150 kg, most of them are obese morbid. In modern rugby there is little props in excess of 150 kg, or even 140 kgs. Most modern props weigh between 105 and 120 kgs.
An example of Defensive Tackle of NFL:
Vince Wilfork (New England Patriots) Height: 1.88 m Weight: 147 kg
In international rugby few players who exceed 150 kgs, Uini Atonio is one of them, but it's shown that players of that weight in modern rugby only can play as impact players, they can't withstand the intense pace for 80 minutes.
Uini Atonio
Height: 1.97 m Weight: 155 kg
American football is very different, the field only measures 50 meters wide, while a rugby field is 70 meters wide on average, that's a HUGE difference. Furthermore rugby is faster, so a 150 kg player can play american football but can't play modern rugby, or at least can't stand in it