It's not just the weight though, but the location of the weight. Different designs - spokes vs. mags, etc. - will affect the location of the mass in the wheel itself. You could have a 16 pound 16" wheel with most of it's mass at the outer edges, and a 20 pound 18" wheel with most of it's bulk near the axle. The heavier wheel (due to the distance of the mass from the center) may very well be easier to spin.