Since the timing is set by the glowplug/fuel/compression, for a given prop, you optimise the correct fuel to air mixture, if you leave all others alone. A lean mixture will ignite earlier than a rich one - it is the same as advancing or retarding the ignition on a spark-ignition engine.
If you put a bigger prop with a higer loading, the engine will take a little bit longer to make a full revolution. If the mixture is set for a perfect ignition timing with a smaller prop, it will ignite too soon and lead to over-heating.

Leaning/enrichening the fuel mixture adjusts the timing on our model engines, that's why they run hotter on a lean mixture setting and cooler (retarded ignition) on a rich mixture setting.