I have always wondered about this.
Why does the ancient age seem much more "EPIC" as compared to late medieval/middle ages.
Why does the ancient armies of the ancient ages (Roman Republic, Roman Empire, Greeks, Macedonian, Persians etc) seem to have a grander scale or more epic armies and battles compared to Middle/ Late Medieval ages?
For comparison, why does the Roman Empire seem to have a very large, spread out and state funded Military whereas in the Late Medieval/Middle ages Kings have to win the support of various noble houses to acquire their armies?
Well, it is not a unique phenomenon on History. Look at the Late Bronze Age, where the Levant would see the rize of the Egyptians, Hittite, Mycenians, forming great empires, only to be destroyed and the whole region entered a period of unstability, ruralism and political deconstruction. The book "1177 BC", by Eric Cline covers this in great details.
My point is : one should not look at History as an evolution, a simple movement to modernity, there has been and will be improvements and fallbacks.
But, for the details, during the Middle Ages, there wasn't such grand armies as there was during Antiquity because of several things : first, obviously, there is the political deconstruction. Africa, Syria and Iberia became parts of the Caliphate, so i'll assume you are not refering to them. In Europe however, the Germanic kingdoms that followed the Roman Empire separated one political area into severals, splitting ressources and manpower. The gravelkind succession did not help to unify the region, and instead actively diminished the power of the kings, wich helped bring about feodality and so even more disunity, with the kingdoms split in principalities.
Also, consider that in Europe, the only sources of manpower sufficient enough to create large armies was in Gaul, Italy, and on the western side of the Rhine. In Northern and Eastern Europe, there was simply not enough infrastructures to support great number of soldiers. Even in Western Europe, infrastructures were severed, and the transition from urbanism to rurality did not help to ensure the fonds necessary to maintain thousands in the field.
Now, fo the Eastern Roman Empire, the disappearance of the large imperial armies is easily explained : it was a choice. They gave up on large armies and instead created the Tagmata system : small armies of professionnal soldiers-peasants that are linked to a certain province where they farm and, if an enemy attacks, they join the Strategos and help defend the Empire. (John F. Haldon, Warfare, state and society in the Byzantine world, 565-1204)
Hope this will be detailled enough !