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 individual players are only on the field for limited periods and therefore really massive players can be employed for the more static areas of engagement.
For professional rugby, players are often chosen on the basis of their size and apparent strength but are then not really expected to work to become significantly stronger. Much strength training in rugby appears to have the aim of generating hypertrophy – increasing muscle size and thus body mass – or of maintaining strength levels rather than seriously exploring the potential for markedly increased power.
Soccer, Australian football and rugby league are continuous-flow type games, whereas rugby and, to a much greater extent, American football are characterised by frequent stoppages and thus require lower levels of aerobic fitness. But I see little evidence that rugby coaches have fully realised the potential this provides to gain a competitive edge by requiring their players, backs and forwards, to seriously train for strength.
I would suggest that, given the development of very well-drilled coordinated defensive lines, the next stage in the evolution of rugby is likely to involve a concentration on the identification of and development of heavy, very mobile players who possess very high-range explosive strength.