You sure you didn't mean the Meiji era (1868-1915)? From what remember of my Jpn history studies, the population was stable at 30,000,000 throughout most of the Tokugawa Shogunate (1600-1868), mostly through a combination of periodic famines, and infanticide. A confluence of Meiji era industrialization and Christian missionaries preaching against infanticide led to the population taking its normal course and doubling within two generations. Part of the pressure to take over Manchuria in the early 1930s was the Japanese equivalent of lebensraum, that the Japanese mostly-farming population needed more arable land to feed itself, the military, and the workers in armament factories, and Japan saw Manchuria as low-hanging fruit for imperialism.
P.S. The population doubled again in the two generations after the war, so that it was around 120,000,000 by the 1990s, but it has stagnated since then, because of a combination of very late marriages and rampant abortion, so that without a turnaround, the Japanese population is set for a rapid decline in the next generation.
The reference was to Commodore Perry.