....General Kenney pointed out that the population of Japan was limited to 30,000,000 by law before the islands were “opened” by Commodore Perry, and in modern times the balance has been supplied primarily by foreign imports..
Probably something to do with the Tokugawa Period, when ag reforms allowed the population to grow rapidly. It is known there were successful efforts to limit the increase in population from around 1700 to the late 1800s.