I have a link I'd like you to read but I'm at my son's home some distance to mine and using my tablet so I'll see if I can remember other threads I had link it to.
The short answer is bigger engine with higher horsepower might very well use less fuel. Think of the RPM's to get up to a given speed, let's say 7 knots, and the larger engine will use less RPM than the smaller engine and probably last longer as the smaller engine will be more stressed (heat) than the larger engine.
My boat is in for refit and repower and is being prepped as I type this to take the new engine and leg as it is a stern drive jobbie. I had to learn a wee bit as I made the switch. First I tried to match HP to HP with Volvo, but Volvo didn't want to play that game. You see my old engine was a 270 HP at the engine but really a 240 measured at the prop, which they do now compared to the days when Jesus was in diapers. The new Volvo engine I was interest in was a V6 but given the weight of my boat it would be under more stress and Volvo didn't want to warranty it under those circumstances.
So I got a larger engine, much larger (semidisplacement hull with shallow v bottom) that is 350 HP at the prop. It is a larger block V8 than my old engine. Part of my reasoning was that at a given speed, again lets say 7 knots, the rpm will be less and thus use less fuel, not more as you might guess because the hp is higher. The RPM's will be significantly lower, heat stress is significantly lower thus the engine will last longer if I don't use it a lot at WOT.
I don't know your circumstance but if you are older you may not care if the engine doesn't last significantly longer. My son knows he's inheriting my boat and I want to save him the task of repowering, really the expense of a rebuild at the very least. The stress on engines is much more significant moving a vessel through water than any land based vehicle engine experiences. Evening doing 75 mph on the interstate is less demanding; if you look at your car's rpm guage when travelling at that speed you'll observe that is only at about the 1/3rd mark of WOT.
When I talk to friends about the stresses of a marine engine I liken it to a very large container truck carrying potting soil (heavy) permantly driving up a very steep hill, that's your boat engine at work.
