Nero,
I think you're thinking of the lightened UR pulleys, they are the same as stock diameter but the Greddy set is an underdrive set so they are a little larger. Unless my eyes are fooling me, maybe I'll double check the diameter just to be sure I'm not talking out my ass.
The pulley set for the RX-7s actually comes with a belt but that's because it underdrives the alternator and the water pump and eliminates the air pump.