A giant ship diesel -- or any diesel for that matter -- relies on the heat of compression to ignite the fuel mixture, so for all intents and purposes, ignition occurs everywhere in the combustion chamber at the same time. With spark ignition, ignition occurs at the sparkplug and then has to travel from there throughout the rest of the combustion chamber. With that single (or double, with twin plugs) point of ignition, the bigger the combustion chamber, the harder it is to burn the whole mixture.