yes the larger the pulley is less it has to spin to create the same amount of power or movement. think of a bus wheel and stock civic wheels (14"), the bus wheel is gigantic and spins slower but covers more ground, where the civic's 14" spins a sh*tload more to cover the same ground. As for the weight, the heavier it is the more power it takes to spin, its the same priniple of having lighter rims. If the rim is light (and big, say 17") it looks good and at the same time takes less to move which means the car can look good and at the same time perform better than one that has the same size but heavier rims.
Basically those pulleys that your asking about are real good. No youre not going to get tons of hp out of them but you will get some.
I hope this helps out...I know I could describe it better...and probably shorter if I had pictures...I like to draw diagrams
