With the current Republican Congress and President, it seems like it has been too difficult to raise the CAFE standards and get more fuel efficient cars manufactured that way. So the EPA is (voluntarily? being forced?) finally reforming the way it figures out the fuel efficiency numbers for production vehicles:

EPA has long been criticized for using outdated standards for evaluating fuel economy that consumer groups, auto industry officials and car buyers say severely overstate mileage by failing to reflect contemporary "real world" driving conditions.

If many cars have lower miles-per-gallon ratings, car manufacturers will need to manufacture smaller, more fuel efficient cars to entice buyers and meet their fuel efficiency quotas:

The potential for dramatically lower fuel mileage estimates also may mean limits on what you may find on showroom floors in years to come. "Companies will be forced to build smaller cars and move away from larger ones," said Jack Nerad, a Kelley Blue Book analyst. "Over the course of time, car companies have essentially subsidized their economy vehicles to make sure that their fleets" meet what are known as CAFE standards, or, Corporate Average Fuel Efficiency standards for their entire lineup, Nerad said. "They kind of artificially lowered the prices of high fuel economy cars."

I don't really believe the automakers when they complain, I think they can produce more efficient vehicles without too many problems and people will be quite happy buying them. If all their vehicles have to be more fuel efficient, they can use some of that research they put into more horsepower in smaller engines into getting better efficiency at the same horsepower.