[MichaelBick]

In general small wheels make for a lighter drive system. They use less gear reduction to get to the speed you want them to be at and are lighter. In addition, you can usually use less stages in your gearbox which means a more efficient drive.

I don't think that 8" wheels are necessary(keep in mind this comes from a team that used 8" wheels on our 2012 robot). You can usually get by with using slightly larger wheels but more of them(still lighter and more efficient). For example in 2010 many teams used 8 or 10 wheel drive with wheels no bigger than 4.5". In 2012 many teams got by with relatively small wheels, an 8wd, and some mechanism to get over the bump(ramps under the robot and or push down wheels). 971 even used 3.5" wheels with a 6wd and their push down skids.