Lets say you have one huge sector and you are at peace. In this case you constantly need to drain the sector's bank to not overflow. Now if you had divided that one huge sector into 4 smaller ones, they will surely fill up later, but you will be able to get more out of them UNLESS you have the influence and you keep draining your one huge mega sector to be not full. You might not always want to spend a 100 influence to keep that huge sector being full and 4 smaller sectors as I said would fill up later in time which could be a potential issue if you get declared war early on, however later, you could get more resources out of them in the case of a war declaration, not only because the defensive war multiplier bonus, but that you dont need to drain them in peace time like the huge sector, because that would fill up much faster, and would probably need constant draining.
Exactly how TheChaosShow said.
For these reasons, I would probably have more smaller sectors, so I dont have to keep draining my "huge sector(s)" and in the event of emergency (i.e. defensive war), I could potentialy drain my smaller sectors for much less