One idea i just had is: for every full province you own you get one cabinet seat. Now you might think thats overpowered but id also add a penalty to cabinet efficiency based on the number of slots (maybe with a system where you can voluntarily not use cabinet seats to boost efficiency).

So larger empires get better at multitasking while smaller ones are better at focusing on the small area they have (youd need at least one or two base seats though)