I'm not entirely sure what expected is - if you take the scrum half player for the all blacks, he's reasonably small - although scrum half is the position for hte smallest player on the field. The reason they weigh more than expected, is most likely to be that muscle weighs a lot more than fat, and many rugby players are incredibly dense in muscle. Also, having a weight advantage is incredibly useful in most fields of rugby - it is a contact sport. To use as much force as possible, you need to have speed and mass - and so a good player is fast - hence the amount of muscle; but also enough weight that you can affect them. Let's assume that in surface area, a butterfly and bullet are close enough. If a butterfly flew into you at the speed of a bullet, you wouldn't be hurt. But if you were shot, the far heavier bullet would seriously hurt you.
Rugby is a game where the more FORCE you have...the better you will do. Force can be defined as Force = mass x acceleration. so if you have the mass behind you...then all you need is the speed to turn that mass into a tackle breaking weapon. Usain bolt weighs about 90 kg...if he ran at me full pace, I could probably take him down but if usain bolt weighed 200kg and could run the same speed...id be killed. so that's one of the reasons. Other benefits are - increased strength (more muscle) - better protection for collisions - harder for people to move you (harder to scrum against, harder to move when pilfering the ball)
