Imperious Leader nailed the last comment.
Japan entered the War with the West with 51 Army Divisions; however 27 where already in use in China. Thirteen of the remaining divisions were facing the Red Army. Japan was willing to head to war with the Allies with ten to eleven availiable divisions!
Japan would quickly start recruiting from it’s 73 million population, but Japan kept the same habit of attacking the Allies with smaller forces.