273 vs 318

It's easier to get better mileage out of a 273 possibly. But mileage has more to do with performance level take my pervious example. 4.10/273 3.55/318 3.23/360 and 2.74/408 now say each make 300hp. I know no one us gonna build a 300 hp 408 but we are for this example.
Technically each of these combos should have similar performance and mileage.
Each of these combos will put similar torque to the ground at same mph (hp) which is passing the same fuel and air again similar mileage. But obviously 408 is gonna be more streetable vs the 273.

Another way of looking at it you either have to put torque into the rear end with gears or torque into the engine by displacement since most don't want to run deeper than 3.55 the bigger the cid for any given hp wins.