Part of the new proposed CAFE rules is that they will be applied on a scale that varies with a vehicle's "footprint", which is defined as its wheelbase x track. Vehicles with larger footprints will be allowed to have lower fuel economy than vehicles with smaller footprints; this is a way to (sort of) account for vehicle size. The NHTSA decided to use a sort of s-shaped curve called a constrained logistic curve, to map the footprint to the fuel economy target.
For cars, here is the proposed set of curves for 2011-2015 (copied from here, the NHTSA Notice For Proposed Rulemaking, p. 278). To get some idea of what this curve does to actual vehicles, I calculated the footprint of four vehicles and put them on the chart.
I think a possible side effect of this regulatory pressure could be that carmakers will increase the wheelbase and track of their vehicles, particularly vehicles with poor inherent fuel economy such as sports cars.
For example, if the BMW added added about 1.5" to its track and 2" to the wheelbase of the 328, it would go from a footprint of 44.8 ft^2 to 47 ft^2, and therefore from a 2015 CAFE requirement of about 37MPG to about 33MPG, or about a 10% reduction.