From what I’ve read it seems that armies in the Greco-Roman period seemed to be on the scale of tens of thousands, while the scale of armies in the Middle Ages is always mentioned as much lower on the scale of a few thousand. What is the main reason for why powers at the time were unable to put together larger armies?
While more can be said, /u/FlavivsAetivs provided a clear explanation here.
In short, larger army sizes require more centralised organisation to provide the funding, logistics and infrastructure to support massive army sizes. The Persian and Roman empires, for example, had the wealth and manpower to raise and maintain armies in excess of 100,000 men.
In comparison, the greater fragmentation of medieval kingdoms meant that it was far more difficult for any individual ruler to maintain a large army for a prolonged period of time. It was certainly possible should enough resources be combined - say, a Crusade - but otherwise most battles in the Middle Ages were indeed smaller in scale.