A faster gearing makes indeed a lower rpm but the engine has to work more to give the power, whith that knowledge you know you can not go that lean.
With a gearing for high rpm the engine can go leaner without loosing power.
On the other hand more rpm means more combustions....

I think the real story is somewhere in the exhaust. The working of the exhaust is optimal with only a smal band in the rpm range, if you are not on it the engine will create a loss.