Size: 4 500 000 square miles
Pop Density: 65 p/m
Population: 300 000 000
Capital Population: 260 000 people
Max army size: 1 500 000 men
You'll notice that out of a massive population of 300 million, only 250 thousand live in the capital. The vast majority of the people live in the wild countryside. And the army size is deceptive. 1 500 000 men is the theoretical roof that your country could sustain. The Imperial army will never come close to 1.5 million, and those soldiers who remain would be spread out across it's vastness.
Out of 1.5 million men, Frankmark, for example, would have a maximum army 225 000 men, before taking into account desertion and attrition. You'd also have to pay all those men. And that's not the personal army of the Queen of Frankmark; most of it comes from the vassals, some of whom would stall or even refuse to ride with their liege.
On another note, what were the Demon Wars exactly? I can guess some things from the title, but I don't want to write anything about it until I know what I'm talking about.