“loophole” implies that regulators are trying to restrict them, but manufacturers are finding ways to work around those restrictions. There is no “loophole” here: CAFE standards are specifically driving manufacturers to produce larger cars.

CAFE standards gradually tighten emissions standards. The problem is that they tighten the standards on smaller cars faster than on larger cars. CAFE are making it harder and harder to make small, compliant vehicles, and easier to produce larger compliant vehicles.

This isn’t a loophole. This is incompetent, counter-productive regulation.