high way numbers are slightly easier to get with a bigger engine because the engine can have a lower rpm with the extra hp to push the car closer to idle.

ex. (not doing ALL the math)

1.6 liter 4 cylinder at 4000 will consume about the same amount of gas as a 3.2 liter at 2000 rpm.
when you add in gearing, fuel control systems, etc then you can draw out the mpg because you can lose hp to add efficiency.

based on epa testing, idling the car through the test is possible do to gearing. the manufacturer absolutely does set gearing to skew those numbers.

our cars really "excel" around town comparatively.