Your thinking of RRM's, it takes a 6" wheel longer to make a complete round per minute than it takes a 8". Look at the ratio, when you go to a 2" larger wheel you step up 2000 SFPM. Its not talking about RPMs its talking about SFPM..
Say the smaller wheel has to make two rounds to stay the same as the larger wheel making one round.
Does this make since (NEWBIE)
Just watch your rpms and you will be fine....