I was surprised to read these 1938 figures (admittedly estimates) of 1938 GDP for the major combatants of World War II.
In round figures, the U.S. had $85 billion, the Germans $42 billion, Britain $28 billion, Italy $9 billion and Japan $6 billion. Japan's GDP is about 7% of the U.S., 14% of Germany's, less than a quarter of Britain's, and less than Italy's.
Yet, most people would agree that Japan was a larger factor in the war than Italy, because the Allies obtained Italy's surrender in September, 1943, and Japan's two years later. And many would agree that Japan's contribution to the war "approximated" (was half or more) of Britain's, and was a decidedly larger fraction of Germany's (one third to one half) than 14%.
For instance, the Japanese army was larger than the British army (and beat equal or larger numbers of Britishers in battles all over Southeast Asia). If you include British "colonial" forces, e.g. Indians and Africans, and Japan's "colonies" in China and Southeast Asia, the numbers of "British" and Japanese population and troops were about the same. The Japanese navy was about the size of the British navy (and actually stronger because of a larger number of modern ships).
Is it reasonable to assume that my linked figures tell the true story, that Japan did the above with "only" one quarter of Britain's GDP? Are there other relative GDP figures that are more reflective of Japan's relative contribution to the war? Or is it more likely that the linked GDP figures for "Japan only" understate Japan's true war potential because the "Japanese Empire" was a lot larger than just Japan? (At its peak, it included half of China, Indochina, Indonesia and the Philippines (basically today's ASEAN countries), Korea, Taiwan and other Pacific countries.)