The big difference is the continuous nature of the game- rather than a serious of short plays, there are two 40-minute halves, during which the ball is in play, and in motion, continuously until it either leaves the field (whether by someone scoring or via a touchline) or a rule is broken that requires the ball to be given to the other team. There are also very limited tactical substitutions, in contrast to American football's platoon system where specialist offensive and defensive teams alternate being on the field.

This means that a rugby player must be able to keep running for the whole match, so they can't be as big as the biggest NFL players. For example the heaviest player on the England rugby team, prop Will Stuart, weighs in at 291 lb (most of his teammates, even those who play the same position, are significantly lighter). Every single offensive lineman on the current Chiefs roster, plus several players in other positions, is heavier than him.

Another effect of the stop-start nature of American football, and in particular the system of downs, is that small differences in how far the ball advances are often very important, and are more important than actually bringing the ball carrier down. In American football, a tackle is a head-on collision between the ball carrier and (often) several defensive players, with the aim of immediately stopping the forward progress of the ball. A rugby tackle is usually one player, often from behind, wrapping his arms around the ball carrier's legs to bring him down, at which point he has to give up the ball but play continues. It's designed to bring him down as efficiently as possible (because play won't stop), while leaving the tackler's teammates free to tackle whoever receives the ball if the ball carrier can get rid of it.