Rugby players spend considerably more playing time in physical contact and contest with opponents than players in other forms of football.
Much of this contact involves extended grappling and wrestling, but what is also characteristic of rugby is the amount of time spent attempting to drive forward under loads considerably heavier than bodyweight. Obviously this is so in the scrum and maul, but also at the tackle. Both ball-carrier and tackler may strive to drive one another backward for an extended time after engagement. American football and rugby league are also primarily collision sports, but their tackles tend to terminate much more quickly.
Recognition of the importance of physical strength has led to a tendency for rugby selectors to favour increasingly heavier players even for backline positions. A modern professional rugby team is likely to average over 100kg bodyweight, compared with less than 95kg and less than 90kg for rugby league and Australian football respectively. Increased bodyweight appears to confer no advantage in soccer.
No valid size comparison can be made with players in American football. Its use of specialist teams means that...