Historical or fictional army sizes are often said to be unrealistically large, but China is sometimes listed as an exception. For example, Bret Devereaux said that medieval European armies listed as being 100,000 or more were unrealistic, with a caveat about some Chinese armies reaching that size.
Everybody I've read saying this isn't a Chinese historian and just references it offhand. I'd like to read someone addressing the issue. Book, blog, whatever. What were the largest well-attested armies? How did they support such large numbers? Were the compositions of the armies different, or something else? Did the nature of the wars or governments involved allow them to field larger armies?
I know basically nothing. Please tell me things, kind historians.
Comparing Medieval China and Medieval Europe, China had about double the population, and for much of the time was united as a single country, while Europe was divided into many countries.
In the 15th century and into the 16th century, Europe's biggest military power was France. How large an army could they field? Francis I invaded Italy in 1525 with an army of about 40,000. This was not France's entire military force, since (a) it would have been dangerous and foolish to leave France undefended, and (b) there isn't much point taking a larger army than can be supported because they will starve or desert. The total French military force at the time might have been over 100,000, and that 40,000 Francis I took with him was a large chunk of that (but almost half of it was foreign mercenaries from Switzerland and Italy).
At the same time, Ming China had a population about 6 times larger than that of France, and a paper strength for its armed forces of over 1,000,000, and real strength probably below but still close to 1 million. Six times the population, and a total force about 6 times the size seems quite realistic. However, the Ming could not put that many soldiers in one place, because they couldn't support them. For example, when the Ming sent an army to suppress a rebellion in the Ordos/Ningxia campaign of 1592, the army was about 40,000 strong. In the same year, Japan invaded Korea, and China sent a force of 40-50,000 for the first round of the war (Korea asked them to send more, but as the Ming explained to them, there was already a major food shortage in Korea due to the war, and sending more soldiers would result in more Koreans starving to death), and about 100,000 for the second round (including large naval forces). At the end of the Ming dynasty, their largest army in the field after the capture of Beijing by rebels was Wu Sangui's army of 100,000 by the Great Wall facing the Manchu army of about 60,000 (when these two armies combined to fight the rebels and recapture/capture Beijing, this was a very large army). The combined Ming-Korean army that fought the Manchus in the Battle of Sarhū (1619) was about 120,000 strong (and about 85-90% Chinese).
The Ming did on some occasions field armies significantly larger than 100,000 (e.g., one campaign against the Mongols involved an army of about 150,000), but this required a major logistical effort - gathering supplies, transporting supplies, etc.
Thus, the total Ming forces were about 6 times the size of the total French forces, in proportion to their populations. However, the did not manage to send forces 6 times the size of what the French did into neighbouring countries - instead, they often sent forces of similar size, and when they sent larger armies, they were usually only 2 to 3 times the size of the army Francis I invaded Italy with.
Japan, with a population similar to that of France, managed to invade Korea with a force of about 160,000 (including sailors and non-combatants). The largest single Japanese army operating in Korea was about 90,000 strong. After the Japanese advance bogged down, and they couldn't readily make further progress against the Chinese-reinforced Koreans, the Japanese forces negotiated a ceasefire and withdrew to the Busan area where they could be supplied by sea from Japan - they had lived off the land in Korea like a swarm of locusts, and they were short of food and had little choice but to withdraw to where they could be fed.
In summary, population is important for determining the total size of a country's military force, and logistics determines how large a force can be gathered into a single army and for how long.