Well there is an argument to be made here.

Generally, cars with smaller engines are lighter - compare a slant 6 to a 318 to a 440. So in this case the "lighter" car would get better milage.

The other piece of this equation is the time to reach a given speed. Generally the smaller, lighter cars will see better economy because they're slower to accelerate whereas the driver of the 440 car probably has a heavier foot - getting into all 4 barrels !!

So the only way to really gauge the economy would be to have all vehicles geared the same, weigh the same and accelerate and maintain the same speed over a given distance.

I strongly suspect the results would be very close.