Romans and many other countries managed to put together tens of thousands of soldiers just to make average sized armies. When facing Hannibal romans lost literally tens of thousands just in one battle, then they got up, assembled a new army and lost 15k men, and again, and again and again.
But when i read about medieval armies they rarely achieve those numbers.
why is that?
As a starting point, you might want to look at the discussions we've had here and here. This is a question I've put a lot of thought into; I keep a running spreadsheet to track various possible factors, like the price of food, population density, urbanization, tax revenue, soldiers' wages, etc. I still don't think I have an answer, though; I'm mostly just able to discount some possible explanations.