Yes battles were usually bigger in classical era (rome, greece, ...), I think 20 000 - 40 000 men on the Battlefield was the norm, with some (famous) battles being far bigger [expl : Cannae 80 00-130 000 depending on the source, Ipsus 140 000 men, 20 000 horses and 600 elephants !)

The medieval armies of western europe could not field so many troops - 10 000 men on each side was already a very big fight at that time, 5 000 is more frequent. This not however because "civilization" has collapsed or population has dropped.

I think the cause is the feudal system itself - no real centralized states, little money to pay professional soldiers for a long time ... Discipline and troop control was also a repeated source of mishaps. And the larger the army, the more needed a strong command system to use th troop with effectiveness.

The domination of (often noble) cavalry during most of the period is also an argument for small armies, as they are costly to train, and large mass of cavalrymen are hard to keep under control (the mongol managed that ... hence their superiority on most enemies).

All what I said is (I think) true for Western and Northern Europe. In the Holy Lands -or later in spain - The treath of powerful enemies seems to have lead both side to greater unity and bigger armies. Several sources described battles with tens of thousands on each side (Byzantine - before Manzikert- fatimid and bagdad califate troops, moors, mongol and crusaders of course).

All you need is a common enemy after all ...