Into the weeds engine design for fuel efficiency discussion.

Phreakish said

Show me an instance where a larger engine gets better mpg for a road going vehicle.
Larger engines create excess power.

Click to expand...

A large engine operated at 3000 rpms with a slight throttle opening will get better mileage than a small engine at the same speed at WOT.
Put a slant six in a one ton truck and try to haul a load up a grade while getting more than 8 mpg.
A 440 can idle up the grade and beat the mileage of the slanty.