James Ryle
Warlord
Japan's population is not that much higher than mine. I have 7 cities they have 6. I've built military buildings in all my cities so it should still not be that much of a discrepancy.At emperor difficulty you get 15% of your pop as supply while AI get 25% of their pop as supply. Maybe that's part of the explanation.
Also they might have built more military buildings and defense than you did.
I don't know if promotions add to war score.