You only pay for the horsepower you use. With the exception of more engine weight and higher internal friction in a higher horsepower engine which has a modest impact on fuel economy it is your right foot that regulates your fuel consumption.

If you have a big engine, but choose to climb long steep hills using the performance of a smaller engine you will consume the same volume of fuel as the smaller engine. But that is not how we drive. If we have a big HP engine and we can pass every vehicle in the slow lane going up hill, that is what we do so we burn the fuel proportional to the HP we used. Hills and starts are what use fuel. Once rolling on level ground we are using horspower to overcome the wind and rolling resistance. But getting a heavy coach moving or going up a hill requires more fuel consumption than a lighter coach.

If you study the charts, a 10,000 pound heavier coach requires 8% more horsepower, but increasing the speed (headwind) 15 mph requires about 57%. Weight is a factor, but speed is the largest impact.

My car has 505 HP, far more than any of my other three cars, but it is the cheapest car to drive with a 24 MPG fuel economy around town, and over 30 MPG on the highway. Up until I stick my right foot on the pedal.