[Concrete]

I believe what he is saying is that a tiny engine running in the middle of the power band is more efficient than a large engine idling along at 1700 or 1800 RPM regardless of whether is gas or diesel.

Ideally we install little lawnmower engines in our cars and run them at +60% throttle in cruise (peak BSFC)
but since most of us don't want to die merging into traffic on the freeway we have over capacity
this over capacity is often over 10 to 1 what is needed in cruise

your engine is rarely at peak BSFC
unless you are climbing, pulling or for the few seconds of acceleration up to speed

hope this is helpful