Rotating mass, does that ring a bell?
I have ran like 4 sets of wheels on my car. The best wheels are 17". They just FEEL better going in and coming out of turns. The responsiveness is bounds better than the 18's and the ride is more plush, but not to the point of "smooth".
First wheels (Toms C7R) 17's:
Second set 18's (ebay special Sportmax):
Latest wheel (current) SSR Vienna Gluck 17's: