Bmx bikes please answer 10 points right away!?

Why do some bikes havef that small chain holder in front. You know where it was teeth for the chain? The one in the front. Why do some have really small ones? Is it a big advantage? Thanks!
Update: yeah the front chain ring where you pedal. Not talking about trial just freestyle. Some are like 8 inches big and some like are like 3 or 4 why??
5 answers 5