i had the same question, so i made myself look like a dum ass and went and asked a biker lol. a smaller chain wheel helps so your pants dont get caught in the chain while riding. it helps with quarter/halfpipes not hitting the coping when you drop in. rails, its easier cause you have more room. it also helps when your in a tight spot to get going faster( smaller chain wheel= less pedaling in that situation. im looking to get a 24 tooth chain wheel personally.
