In 2008 CAFE introduced a formula that calculates the required MPG based off of the track times the wheelbase (basically the rectangle formed by the tires). Trucks are less aerodynamic than sedans, so small trucks can't be made that get sufficiently low MPG to meet this standard, and thus even though a smaller truck would use less fuel than a larger truck that meets the standard.
TL;DW:
In 2008 CAFE introduced a formula that calculates the required MPG based off of the track times the wheelbase (basically the rectangle formed by the tires). Trucks are less aerodynamic than sedans, so small trucks can't be made that get sufficiently low MPG to meet this standard, and thus even though a smaller truck would use less fuel than a larger truck that meets the standard.