low rpm means you might get more of the energy out of the fuel. smaller engine means lighter car (and once we remember basic physics, lighter cars get better mileage).
back to an earlier question, but lets put some real numbers to it. Lets say our car needs say 22 horsepower at the wheels to go 70.
Are we better off with a 500cc engine turning 4000 rpm, or a 1500cc engine running 1300 rpm?
both numbers assuming 10:1 run from
Rough Engine Horsepower Determination Program
FWIW< the calculator is shockingly accurate as an upper goal.