Martin Vargic made a very interesting chart that shows what the world's largest army was throughout history, from 2500 BC until now. He split up the world into "west" and "east" to show which countries or dynasties or empires were dominating different parts of the world.
It's interesting to compare eras with each other and see what having the largest army meant (in relation to history), since it was basically a numbers game. Though our militaries are more advanced now, there was much more manpower back then. I think the most fascinating thing is how large the Ming Dynasty army was compared to anything in their time.