Low RPM = More Fuel?

[bcurling]

I was told that running a larger motor (15hp at half throttle to get 7kts which is the hull speed for a particular boat), will use more fuel than the intented design motor of 9.9hp at full throttle. I always thought that using less throttle to achieve a certain speed would consume less fuel than a smaller motor at full throttle. Am I off base on this? Someone please set me straight on this.