Its conventional wisdom among BMXers to run a fat tire up front and a skinnier tire in the rear. BMX races consist of short straights with jumps and a number of extremely tight banked corners, all on dirt. I've always thought that the big tire up front gives better handling and traction through the turns. I've never actually raced BMX..I was more into freestyle. At any rate, I suppose that would make sense for conditions other than dirt as well.