It's not always true that the smallest cars get the best gas mileage. In many cases, compact cars get mileage equal to or better mileage than their shorter, stubbier subcompact brethren.
The new 2012 Chevrolet Sonic subcompact has managed to crack the magic 40-mpg highway mark, though. Estimates for the version with the 1.4-liter turbocharged engine and six-speed manual transmission are 29 mpg city, 40 mpg highway, for a likely combined EPA rating of 32 mpg.
Lower-level Sonics with the 1.8-liter engine are rated at 25 city, 35 highway, and 28 mpg combined (with the six-speed automatic), or slightly better with the five-speed manual transmission: 26 city, 35 highway, and 29 mpg combined.
But to prove the point, the 2012 Chevy Cruze Eco compact, which has the same engine and transmission but is a full car class larger, gets 28 mpg city and 42 mpg highway, for a combined rating of 33 mpg.
Some of the difference is due to aerodynamics: It's easier to make a longer car slip smoothly through the air. And these days, subcompacts are almost as heavy as compacts, since they must have complex steel-cage crash structures and as many airbags as larger cars to do well on stringent safety tests.
2012 Chevrolet Sonic LTZ with 1.4-liter turbocharged engine
In the end, whether you choose a compact or a subcompact may come down to which car size you prefer, whether you like hatchbacks or sedans (Chevy isn't offering the Cruze hatchback that it sells elsewhere in the U.S. market), and how much space you have to park.
The 2012 Sonic, in any case, now has equivalent highway gas mileage ratings to the 2011 Ford Fiesta SFE model and all models of the Hyundai Elantra. So far, no models of the Toyota Yaris or Honda Fit have cracked that barrier.