Into the weeds engine design for fuel efficiency discussion.

Nope, smaller cu in doesn't necessarily mean better economy. Every engine has a 'sweet spot',
where all it's parts work together to give best mileage or best power, & rarely both....
A small engine has to lug. A big one doesn't.

It is very difficult to give an accurate answer to the original question because of so many variables, other than use a carb with smaller pri bores [ eg, a TQ ] & a high velocity dual plane intake. Good ign system like HEI that will fire a 060 plug gap.

I see a lot about piston crevice volume [ CV ]. Some people even use it in their CR calculations! Need to think why there is a CV: the top of the piston runs hotter where the rings are because it is closer to the combustion heat. If the piston diam was not reduced in this area [ CV ], it would seize as it heats up. The CV is all but gone at operating temp.