Normal sized wheels and tires don't offer the high final drive ratio of those tiny ones, giving the car a massive acceleration boost with negligible traction loss, both of which are very important in the short bursts between corners on an autocross course. Also, welcomed reduction in unsprung weight from the wheels and tires.
I'm glad he didn't put bigger wheels/tires on the front like some people do. In my opinion thats a last resort for when the mechanicals of your car simply won't do what you want them to do.