The population of Japan and France in 1600 are both losely estimated at 20 million. But Japan is 20% to 30% smaller than France and France is often considered to have a high pop per area compared to other European countries. How come Japan's population was so high? (considering that the most prosperous period had yet to come)
But even for such a population, Japan could still field enormous armies. Daimyo could field armies in the ten thousands and the battle of Sekigahara had 80 thousand soldiers on each side. Why is it possible that Japan could field such enormous armies ?
I would say No. Japan did not field a higher percentage of the population than European countries.
Working off of numbers from Wiki for Europe (I am not familiar with European numbers so if anyone has anything else please post them) at the Battle of Towton (1461), with an estimated 3.3 million population in England and Wales and an estimate combined force of 60,000, 1.82% of the population was at the battle. At Lutzen (1632) the Swedish Army brought 19,000 to far away Germany. That is 1.27% of the Swedish population if we assume 1,500,000. 0.95% if we assume 2,000,000. And 0.76% if we assume 2,500,000.
Compared to that, assuming 160,000 at Sekigahara (1600) and 250,000 at Osaka (1614-15), the percentage is 0.80% and 1.25% respectively. Even if we go for the absolute maximum estimate for Osaka of 300,000, that’s still only 1.50%.
In the late Sengoku Japan, a daimyo fielded a maximum of 60 men per 1000 koku (based on Uesugi Kagekatsu and Akechi Mitsuhide’s muster orders in 1575 and 1581 respectively). The usual number is between 30 to 50, with most daimyos bringing 30~35 men per 1000 koku to Sekigahara (there are outliers both ways, but I’m not going to take them into account here). Working from those numbers and estimating from the survey records of 1598, Japan could field a maximum of about 1.1 million troops for 60 men per 1000, or about 5.5%. At 40 men per 1000, the overall mobilization would be about 740,000 men, which is 3.7% of 20 million. 700,000 to 750,000 are about the range of estimate I have seen for the number of men under arms in the closing days of the Sengoku.
In comparison (again working off Wiki numbers), the Spanish Empire had 300,000 men under arms with a population of about 12.5m (2.4%) for its European territories, or 8.25m (3.6%) for just the Kingdom of Spain. The Dutch Republic, with 1.5m population had 77,000 men under arms (5.1%). Sweden with 2,500,000 had 149,000 men (6.0%). Denmark-Norway, with only 1m population at 1600 had 135,000 men under arms in 1625 for a whopping 13.5%
Sorry for having to use Wiki numbers for Europe. I eagerly welcome other numbers so I can actually say with more certainty Europe’s military capacity of the time. If someone has a source that lists, say, the population and army size of each participant of the Thirty Years War at the outbreak and conclusion of war, I'd gladly edit my post to reflect those numbers. But as of right now I see no significant difference in mobilization as a percentage of population for Europe and Japan. Of course, despite my flair I am actually biased against any type of Japanese exceptionalism that's not backed by primary sources, so take what I say with caution.
As for why Japan had such a large population, someone else would have to answer that.
EDIT: Taking out Uesugi because on an inkling I double checked my calculations and found them problematic at best. Sincere apologies.