For a tax to be effective for such a purpose, it has to be avoidable. They have to actually make a small car. But the CAFE standards as they currently stand prevent them from cheaply producing a CAFE compliant small car. So nobody gets the tax break on the small car, because there are no small cars to be had.
The tax approach cannot be achieved until the CAFE standards are fixed, but once we fix the CAFE standards to favor smaller cars, the problem solves itself.
CAFE works by requiring a certain percentage of the total number of a manufacturer's vehicles to comply. Small cars are currently non-compliant. Only big cars are compliant, so they need to sell more of them. When we correct CAFE standards to favor small cars, they will need to sell small cars, and their marketing departments will get to work at adjusting consumer demand.