system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were armies in the Middle Ages smaller than those in ancient times?
gpt
Because from the fall of the Roman Empire onwards, Europe saw the rise of a system of governance called Feudalism, rather than the state-based systems of the Roman, Makedonian or Assyrian Empires, for example. 

A feudal system is one in which, instead of the government being absolute sovereign of the country they govern, the king is placed at the top of a heirarchy of nobles and officials each with responsibility for their own territory *within* the kings domain. So the king of France may be 'de facto' king of all of France, but in the area around Orleans, the Duke of Orleans is absolute sovereign and there isn't anything that the King of France can do with Orleans or the people there. 

The King is only the King because each of the nobles swear alliegence to that king. That means they'll usually provide him some money, and will fight for him if war breaks out. In return, the king doesn't mess around with their territory. Because of this, the king typically only owns a small assortment of towns or even just his own palace. (His demesne) and so can't raise an army of his own. Instead, he'll entrust his nobles to raise their own armies out of the villagers in their individual areas. 

This means armies can be raised quickly when war breaks out, but because there is no single 'State Army' which owes allegience to the government, the armies are usually much smaller and under the control of many people. The lack of a centralised governing authority would make it completely impossible to clothe, feed and pay an army of Roman sizes in the middle ages. 

Also, throughout the middle ages a huge amount of fighting was done by mercenaries, because the state couldn't train its own soldiers, people who already knew how to fight were in high demand. Because demand was so high, prices were too. It just wasn't practical or affordable to pay a Roman-sized mercenary army.