It's a few things all adding up:
- Cars are getting heavier which means you get a bigger engine, etc. - Tighter emission controls cost energy - The EPA changed their standards in 2008 to reflect a more realistic driving mix
When Honda first introduced the accord in the late 70s, it weighed about 2000# and had a 1.6L engine and was rated at 45mpg highway; in 1981 they switched to a 1.8L; 1986 got the 2.0L; 1990 went to 2.2L; 1998 was a 2.3L; 2003 saw the 2.4L. That's 50% bigger than the original, and the curb weight for the Accord is now around 3400# and rates 30mpg/highway.
Curb weight for a 2010 VW "Rabbit" is nearly 3000# -- though the TDI (diesel) engine is still king of the road, currently spec'd to 45mpg.
/jordan