While many factors contribute to an engines efficiency, the primary factor is the engine geometry itself. Not only the overall size of the engine, but also the aspect ratio of the engine cylinders, defined by the stroke-to-bore ratio.
An engine cylinder with a longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gasses compared to a cylinder with a shorter stroke-to-bore ratio, leading directly to reduced in-cylinder heat transfer, increased energy transfer to the crankshaft and, therefore, higher efficiency. As the stroke-to-bore ratio increases, so does the distance fresh air has to travel between a cylinders intake and exhaust ports. This increased distance results in higher scavenging efficiency and lower pumping work.
As the stroke-to-bore ratio decreases, bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings. However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface.
Engines that require high fuel-efficiency, such as large ocean-going vessels, have a large stroke-to-bore ratio and, owing to the inertial forces of the piston, requires a slower engine speed and lower power density.