thus it requires a certain amount of fuel to create that horsepower
That's only the BTU content of the fuel. All the combustion does is expand the working fluid, and except for diesels the cylinder is never full at part throttle. What's different is that anything that increases the working fluid mass improves power and reduces pumping loss. An engine at 14.7 can't get the same mileage as one with 60° spark and 22:1 mix. The same fuel volume is producing much more power, but those settings are very knock-sensitive and can't be used at WOT.
