The demon world is very complex and extremely hard to deal with, the size of its landmark is similar to that of The Silviya realm, each side having the same land mass, yet such a large place has only 2 empires? yes that's the truth but there is something deeper, its the fact that each part of the Empire is divided into Dukedoms that are the size of a country.
And the rulers of that country are called kings but in the end, the one at the top will always be the Emperor of the Demon Empire, for this, we could take the example of the vampire king and the werewolf king both of them were kings ruling over a large dukedom, in fact, there's were the biggest and the most powerful.