Smaller wheels also let you set your whole robot down lower to the ground (low center of gravity, yay!) and they allow you to make your wheelbase about 2" longer than one with 6" wheels (less tippy, yay!). There’s lots of good reasons to use them.
We put 4" wheels on our robot last year since it didn’t have to do any climbing, and I didn’t notice any difference in performance between it and our previous year’s robot (with 6" wheels). Both were set up with 6 wheel drive and wedgetop tread.
I’ve also been kicking around a wooden eight wheel drive base in my head for a while, but it looks quite a bit different… I like the eight wheel drive concept, it has several things going for it that make it preferable over six wheel drive (IMO). Having four center wheels instead of only two slows down tread wear, and as someone else stated doesn’t let your robot “rock” while it’s driving around. It also gives you a bigger contact patch on the ground to avoid being spun. By bigger contact patch I don’t mean more traction, but traction where you need it. Six wheel drive bots spin pretty easily when pushed on a corner since all their weight is on their two center wheels. But an eight wheel drivebase has four center wheels that carry the weight, and they’re all distributed further out from the center.
And the obvious reason for using an eight wheel drivebase is that it allows you to climb over all those ramps on the field without high-centering.