Well they do that now and have been for the last 12 years, you know. It's called CAFE and the "gas guzzler" tax.
Actually, the Corporate Average Fuel Economy standard is based on strictly fuel economy only. In Europe and Japan, they impose excise taxes based upon the size of the vehicle and engine size; if they did that in the USA the result will be sales of cars like the Honda Civic, Toyota Corolla and Ford Focus will zoom through the roof as customers go out of their way to avoid the excise taxes.