Furthermore, in order to obtain the same level of torque in a single-cylinder that a multi-cylinder engine creates you'd have to account for the power generated per cylinder.
In the Wartsila RTA96C you have HP up to 109,000 (rounded) and torque up to 5,600,000ft-lbs [1]. These #s are for all cylinders. Assuming it is the 14-cylinder variant generating this power and you want to reduce this down to a single-cylinder package then you can easily deduce that the displacement would be around 25,480 liters (mentioned elsewhere in this thread).
Assuming a stroke to liter ratio of 1.374mm/liter (2500mm stroke for each 1,820 liter cylinder) you'd end up with a 35,009mm stroke for a single-cylinder engine vs. a 14-cylinder engine. Since there are 25.4mm in an inch that's a 98-inch stroke for 14-cylinders vs. a 1,378-inch stroke for a single-cylinder or an 8-foot stroke vs. a 114-ft stroke.
Finally, since you'll need a piston many times larger than the current setup for a single-cylinder vs. a 14-cylinder engine then you can imagine how much mass you have moving which can do some serious damage if it becomes unbalanced. Also you'd have an engine that instead of being long and relatively low is now very short and extremely tall (not something you want in a ship where capsizing is a very real concern).
[1] http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96...
Theoretically, the 14-cylinder Wärtsilä-Sulzer RTA96-C could be even more efficient if it were instead a gigantic single cylinder. There must be some other engineering or economic limitation which prevents us from manufacturing 25,000 liter cylinders.
edit: example: http://en.wikipedia.org/wiki/Square-cube_law#Engineering_exa... "Watt recognized the problem as being related to the square-cube law, in that the surface area of the model's cylinder surface to volume ratio was greater than the much larger commercial engines, leading to excessive heat loss"
edit2: There is also a square-cube law with aerodynamic losses related to piston speed (how fast the air gets sucked into the engine: "pumping losses"). The slower you spin the engine, the lower your pumping losses are. However, I believe this plays a smaller role in affecting efficiency than the heat losses. (also, this engine is turbocharged, which changes things a bit).