Larger engines don't necessarily mean more fuel plus he's looking for a power mpg balance, say it takes 20 hp to move your car down the road, it's gonna take a narrow range of fuel and air to make that 20 hp, so it depends on how efficiently you can make 20 hp at part throttle neither engine 318 vs 360 is using vastly different amounts of fuel and air is why you don't see huge differences in mileage.
