WASHINGTON -- The tough fuel-economy standards that took effect in 2012 are getting tougher every year. So why are cars getting bigger?

The average new vehicle's "footprint" -- the rectangle formed by its wheelbase and track width -- hit a record 49.9 square feet in the 2015 model year, according to the EPA, up by about 1 square foot, or 2 percent, since the agency been tracking the measure in 2008.

The EPA says that growth mostly reflects shifting sales toward trucks and SUVs. And to be sure, cars and trucks have been growing for decades to reflect the visual tastes of designers, the safety concerns of engineers and consumer desire for more interior space.

But these days, analysts say, automakers have an added incentive to make their cars a little bit larger: more forgiving fuel economy targets.

The Obama administration's so-called National Program assigns mpg and CO2 emission targets to vehicles based on their footprint, and requires annual improvements for every footprint size. They're also set on curves, one for cars and another for light trucks, in which smaller vehicles face more stringent targets than larger ones.

For manufacturers, that means adding just a few inches to a vehicle's wheelbase during a redesign can result in a lower mpg target -- sometimes 1 to 2 mpg lower -- than if the vehicle had stayed the same size. The redesigned vehicles must still get better fuel economy than their predecessors, but don't need to stretch as far to meet their targets.

"Cars are changing their dimensions in order to take advantage of this, but I don't think it's anything new," said Dave Sullivan, an analyst at AutoPacific Inc. "We've been using every available credit or loophole that the system allows for years now."