The federal fuel economy standards have a weird calculation that incentives vehicles with a large footprint. It's the ratio of footprint to MPG that's important to the feds.
In EU car makers are paying for extra g of CO2/km on every vehicle. And that g/km is calculated based on weight of the vehicle. I don't have exact numbers, but basically 2 ton SUV can have something like 140g/km without penalty, while 1.3 ton hatch has to have 70g/km which is impossible.