Fuel mileage is a strange animal. Like mentioned above, gear ratios will make a big difference as does weight and expecially aerodynamics.

However, as for the engine, fuel economy comes from the enignes ability to run lean and still makes good torque/power at throttle positions under 20%. High compression ratios help quite a bit.

Ultimately, teh city numbers are almost the same. THe big difference is in the highway numbers. I suspect that although the RSX does not have a significantly better coeffecient of drag, it does have a slightly smaller frontal area. It's a smaller car than the Civic.