Actually, there are plenty of slightly pudgy players that happen to be very good rugby players. They tend to play in the front row, and every team tends to need two if not three good ones. Rugby front rows need to bulk up within reason, and be fairly short for a low centre of gravity to do the scrummaging work effectively, while the locks are usually six foot plus for lineout work, although increasingly back row forwards also start to get involved with lineout work in the modern game, with plenty of locks and back rowers able to play several positions and with the height to cope.
Wingers tend to be small fast and nippy although the modern game has produced several notable big guys out wide, Lomu being the obvious but Ben Cohen and Lote Tuqiri are others at the top of their profession and big blokes on the wing.
Scrumhalves tend to be tiny again to exploit gaps and to get down to the ball quicker, but aside from that the rest of the back row tends to vary in size. Centres tend to come in various varieties, from the crafters - Olly Barkley and Brian O'Driscoll are both fairly small - to the bruisers - can anyone say Mike Tindall?