Numbers, yes, to a degree. My stereotyped understanding is something like:
Groups on the steppe tended to have low numbers, because of the low intensity of subsistence. At the same time, they could have high mobilisation, because of the same factor (subsistence doesn't take as much time / effort away) and because they spend a lot of time fighting each other, are well trained.
So steppe groups could field pretty good sized armies against intensive agriculture sedentary states. Even though sedentary states had a large pure population advantage, it was hard to translate that advantage into actual trained fighting strength.
At some point, sedentary states begin to be efficient enough at producing surpluses and raising tax revenues that they can field armies that are just better, both by being larger and having better trained, superior soldiers and strategists, man-for-man, unit-for-unit. This is most true for states with high per capita productivity and that can tax at higher levels.
The trends with mounted archery and gunpowder will tend to exaggerate these, as the former plays to steppe nomads strengths', and the latter plays to sedentary logistics advantage. But it's lots of the above happening at the same time as gunpowder as well.