[ProDigit]

It all depends on the load.
The engine revving at 4 k rpm, 3 cylinder has a similar load as a same single cylinder size, 4 cylinder revving at 3k rpm, similar mpg results.
The weight savings of the smaller engine only apply when there's a lot of acceleration going on. Yet even then, the engine needs to rev higher to get similar acceleration, as the load (percentage wise) is higher on a smaller engine (still needs to push 4 tires, driver, passenger, luggage, body, gearbox).

Smaller engine means lower HP, lower speed, lower weight, and also means you can tune the car. Smaller engine compartment, smaller brakes, smaller wheels....

Nothing affects mpg, as final gear ratio.
2 identical cars, one with a direct drive as final gear, and one as an overdrive, the one with overdrive will do better in mpg.
Which is why the Smart for two has such a low mpg, because it revs quite high in final gear.

A 2CV with twin boxer engine supposedly got twice the mpg of current econo-cars (80mpg vs 40mpg on modern economy cars), all thanks to it's 350-500cc twin cylinder equipped with a large overdrive. The thing could only do 45mph..

The best mpg, you get from an as small engine as possible, tuned to the lowest rpm range possible, at the speed most frequently ridden.
A turbo charged, ecotec car makes most sense for occasional acceleration, but mostly continuous speeds (eg:highway speeds of up to 55mph). The turbo is just there for the acceleration phase, after which the engine reaches it's desired speed. At that speed, the rpm should be lower than needed for the turbo to be triggered, to essentially ride at a lower overall compression ratio than a NA engine, which is less efficient... But more efficient than riding with turbo working...

Also, compared to a larger size engine, a turbo during acceleration phase, negates the fuel gains of the smaller engine size idling at a stoplight.
So ecotec engines make no sense in start-stop traffic.
They neither make sense at the point of saturation (where the turbo robs the engine of hp at high rpm).
And neither at a constant speed, where the engine will run at a lower psi, and efficiency, than the same engine without turbo.

Turbo engines have a very narrow rpm range where they are functional.
NA engines are more fuel efficient, especially with direct injection. A 1 liter engine usually is geared to work well in city traffic of 35mph, but are less efficient than a larger engine at higher speeds (say, a constant 60 mph)