http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engineLook at the P-V diagram in Figure 2. The difference between the power loop area and pumping loop area is the engine power output. Greater throttle open at 2200 rpm compared to smaller throttle opening at 3000 will result in less pumping loss (smaller pumping loop on the diagram). Granted, there is more to the story (fuel and spark curves and mechanical losses for example) but this is the major reason OD helps mileage. It's also the major reason diesel engines get better mileage. Diesels have very small pumping loops at all operating conditions because they are not throttled. It is also the major reason small engines get better mileage than large ones. The small engine will require greater throttle opening and therefore have less pumping loss.
If you are having difficulty with the pumping loss concept, consider that pressure in the crankcase is 15psi or 1 atmosphere while pressure in your intake manifold is much lower at part throttle, let's say it's 5psi. The piston has to push against a 10 psi differential on the intake stroke. This is pumping loss.
Have you ever noticed how letting off the gas doesn't slow a diesel (without a jake brake) vehicle as much as it does a gasoline? That's becasue the pumping loss is much greater in the gasoline engine.
