Different economics, populations, fighting styles, time periods. They probably estimate army size by time period sometimes.
Different economics, populations, fighting styles, time periods. They probably estimate army size by time period sometimes.
Plus, in Medieval times, people were paid to wage war, therefore keeping a large army was extremely expensive, and often you did not keep an army on your own but hired mercenaries to wage war with/for you. In Antiquity, I believe the reward system was different.
The population densities were, it should be noted, not that different in the two periods. It was not until the mid-1700s that we started to see sustained significant improvements in the carrying capacity of the land thanks to agricultural improvements in England. Until then the carrying capacity would vary, higher during a warm period in the 1200s, falling significantly when things cooled down again in the 1300s.
See http://en.wikipedia.org/wiki/Great_Famine_of_1315%E2%80%9313... to get a sense of how much worse life was in the 1300s than the 1200s.
Most fighting inside Europe was done with 5-20K armies during 400-1500. It started with the Romans. As they faced rebel "Romans" ever more frequently, they faced similarly equipped and trained men so results of big battles were unpredictable. They would favor skirmishes over big decisive battles. And if your fighting is indecisive, why field large armies at great expense?
The well trained, well armed foot soldiers, "man of arms" was equivalent to a Roman legionnaire and formed the basis of these armies. At the time a full suit of mail armor cost as much as a Ferrari today. Now this is an issue of economics because there were few complaints about the price of similar kit from Roman times.
Well trained and equipped armies could face numerically superior "peasant uprisings" with predictable results, just like in Roman times. Romans faced a lot of poorly armed opponents in Europe.
BUT it's hard to say what these trends were driven by. There was a lot of traditionalism around warfare.
Macedonian armies were actually poorly equipped early on. They used long pikes instead of reasonable spears like the Greeks. Macedonians wore rags for armor, while some Greeks they faced wore 70 lb almost full body armor. And similarly in Europe, states started fielding cheap pikemen combined with crossbowmen, archers, muskets and a few well trained melee fighters.
Such armies could stand up to more expensive Ferrari suited, trained all their lives, born into a higher class men. It was probably the noise of firearms that changed military traditionalism. But handheld firearms of the period were mostly noise and smoke. And as Macedonians demonstrate not really necessary to face better armed and trained opponents.
But I don't think any historian could untangle all the factors without a Matrix level simulator, complex systems and all that.