You should check diplomacy tab in kingdom. You can see the number of units each kingdom has. Aserai is expected to have many units because they have a large territory and it is pretty safe so they can focus their armies in one exit.
Well also, in my campaign, I am the Sultan of Aserai and we control the whole Aserai, Western Empire, Southern Empire lands consisting of 19 cities and many castles. Everytime we fight against Khuzaits they do the same to us. It is like their source of human does not run out. In diplomacy tab it is clear that we have more units but idk, maybe it is just how it feels to me.