I mentioned something in my last post about using a simpler system for larger groups of players. This is something I'm very conscious of as a GM as I'm of the mind that the story should flow, that the game should progress at a steady rate, or at least a rate that is comfortable to both GM and players, and that the game shouldn't stall.
The more players in a group, the simpler I want the system to be. I use Dragon Warriors as it's fast and loose and suits the requirements of a large group; I've noticed that with more complicated system, the larger the group the more agitated they get when dealing with an encounter or combat. If there is too much time spent on determining an outcome with a single player the others may get a little impatient, even bored, and there's nothing worse than waiting for your turn only to roll your dice, be told you failed and then be left alone again for another ten minutes. It can be frustrating and more than a little annoying.
So, I favour simpler rules systems for larger groups, and more complicated system for smaller, more focused groups. Here's my rough system guide to group sizes (at least, systems that I've played):
7 + players: Fighting Fantasy, RISUS, *ahem* SKETCH.
5 - 6 Players: Dragon Warriors, D6 System (mainly Star Wars 1st Edition), Basic D&D.
2 - 4 Players: D&D 3.x/Pathfinder, Warhammer FRP (1st Edition), Call of Cthulhu, MERP (at a push).
1 Player: MERP/Rolemaster.