This is not the case... However, just because the wheel and tire is lighter doesn't mean you're automatically going to have less losses and an increase in performance due to weight alone (not taking into consideration grip, contact patch, compound, sidewall flex, etc). What really matters when it comes to wheels is the mass moment of inertia. Essentially, it's how much energy it takes to revolve an object around an axis. Basically, the farther away the mass is away from the center of an axis, the harder it is going to be to rotate that object. Although your 18" wheel may weigh the same or less as a steel wheel, your tire for the 18" wheel may weigh more than the one for the 15" wheel causing more mass to be farther away from the center. Thus, an 18" wheel vs. a 15" in wheel with the same overall tire size may have a larger mass moment of inertia than the 15" wheel.