"....so it isn't so much size as mass that takes the HP."
Its not so much weight, its the location of the weight at the outer wheel. In essence, the further out the weight, the more torque it take to turn that weight at the same efficiency/speed. Think of it this way, off the hub a 100 pound rim at 8 inches will be easier to spin than a 50 pound, 40 inch rim. Its called spinning mass.
Just saying

Not that we are talking dramatic numbers here anyway. But if you drive a lot, travel a lot, and race a lot it may be best to keep then at 18 or under....