Petroleum is a critical resource and most countries manage its use in automobiles for fuel in a fairly direct way: They tax the living shit out of gasoline and diesel. If you use more, you pay more. You have incentive to buy a smaller, more basic car because they will ultimately cost you less.
But not here. We gave exceptions on light trucks because that's all the big three could competently produce back when fuel economy standards were being introduced and we were getting our brains beaten in by the Japanese. Making more fuel-efficient vehicles requires R&D, and R&D spend is money not sent to a shareholder. Since the bigger vehicles don't count towards CAFE and smaller vehicles do, guess which vehicles the big three push harder to consumers?