It is so easy (and fun, if you are engineer or simply technically inclined) to go through calculating how fast your boat is, how many GPH (LPH) engine/boat uses, etc, especially when everything else on the boat is "fixed" and there are no no new tasks currently.
But, let's sum up some things which members have already said in this conversation:
1! If you are recreational user, fuel consumption is almost irrelevant compared to costs of boat slip, maintenance, and other boat involved costs.
2. If you are fishing, and have a big engine, it will probably be too fast even at low RPM for trawling (prop is most often optimized for correct top RPM).
3. With big engine going slow, you are paying for taking big weight of that engine all the time (and huge amount of usable space is taken by the engine). Oh, and working around the engine is "crampier", too.
4. The bigger will last longer before needing to overhaul (probably questionable if you are recreational user since at that duty cycle all engines can outlast you if used regularly or will corrode much before overhaul period).
I will also add this two:
4. If you have a big engine used for slow speeds, how much will you pay for higher amounts of oil, antifreeze, more expensive filters, spares in routine maintenance and taxes (this last one is especially considerable in Europe)? How much fuel could have been bought instead?
5. If you use the engine until it is time to overhaul/re-power, how much does it cost and how fast can it be done for big engines compared to small engines?
Although, I obviously vote for smaller engines (not too small though), mind that I do have a SD boat and big engine which enables me to go at higher speeds if I need to, but I have never needed to, I have only wanted to, and when I travel further, higher speeds become irrational due to fuel costs. Oh well, once on the river I was bored to wait for the array of barges to pass and then I went to planning speed to to go around behind them and I have reached on the other side at almost the same time as the guy in FD boat who has waited for the barges to pass, well that was really efficient.
And then, there is that famous sentence "for running out from storm". The only ones I know that have run out from the storm were the ones that did not came out on the water because of the weather forecast, everybody else on the water had to deal with the storm.
So, according to my experience, the most efficient selection of your new boat comes from the early decision at which one and only cruising speed you plan to travel and to have an "adequate" engine for that. And adequate is the one with which the boat is not underpowered (to deal with the storm, not to run out from the storm). Overpowered seems just to add up the costs and give that smile to the captain when reaching higher speed. You decide what is the worth of that smile.