All being equal, the smaller motor will have lower Io (no load current), and the bigger motor will have lower RM, for the same maximum efficiency.
The current at which they hit that peak efficiency will differ with size though, with the bigger motor being more efficient at high current and the smaller motor being more efficient at low current, so which is more efficient for you depends on your setup, what you want from it and how you use it.
Unfortunately as far as I know only Lehner publish efficiency graphs for their motors, while it is not apples to apples due to the differences between a TP and a Lehner, you can pic a Lehner of similar mass and Kv and look at it's graph to get you within a ballpark of where the efficiency peak will be.
Or you can do as most do and lean on the experience of others, there is a lot of people here that have run a lot of motors in a lot of boats that can give you advice, but they need info for that advice to be meaningful, provide as much of it as possible, the more you can tell us about your boat, power system, intended use, and driving style, the more chance you have of getting a legitimate answer.
Paul Upton-Taylor, Greased Weasel Racing.
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