wheel diameter:8" or 9"?

[67mopar]

i notice some people use 8" diamter wheels and other use 9". why is this? is it because with the smaller wheel, theres a less of a chance of it hitting something because there would be more tire between it and the ground?

[jherbie28]

the 8" wheel is ideal for motocross racing

the 9" wheel is usualy(sp) used for xc racing