bigger diameter piping will increase the amount of air flow but will slow them down . . same as water pipe big pipe will have slow flow while small pipe will have fast flowing what we want is high velocity of air and not only big volume . .

as for cams it depends on the profiles of the cams. . high lift high duration cams will improve air flow in higher RPM hence make power . .

this is my novice explaination im sure the mod here will give you a better picture