I am not certain where you get your information. It is estimated that the Japanese empire killed somewhere between 30 to 40 million Non combatants. The Germans killed about 12 million in the Concentration camps.
The Chinese (both Nationalist and Communist) fabricated a lot of numbers to get assistance from the West. I get my information from people who lived under Japanese occupation. Lee Kuan Yew, prime minister of Singapore, described the Japanese policy as more or less directed towards pacification. There would be a few exemplary massacres of people suspected of involvement in resistance movements, but the general idea was that everyone would learn Japanese ways and, in time, become Japanese. I know of half-a-dozen or so Chinese who lived under Japanese occupation. None knew of anyone who was merely minding his own business who was actually executed. In contrast, they all know of someone who died of hunger during the Communist famines of the Great Leap Forward and of landlords who were executed in the immediate aftermath of the Communist victory in 1949.
Nazi Germany was a unique animal. Empires traditionally aren't sectarian. The Romans added foreign gods to their pantheon and employed Romanized soldiers and administrators from conquered lands outside of the Italian peninsula as a means of knitting the empire together. The Germans decided they did not need non-Aryans in the empire and decided to kill them all.
And then there is Unit 741. It made what the Germans did to the Jews, including the Germans using the skin of dead Jews as lampshades, look like a Cub Scout art project.
Describing what Unit 741 did as medieval would be unfair to the folks who came up with the thumbscrew. They set a new standard.