The United States was considerably richer and more industrialized in 1940 than Japan.

So for starters, the population estimate for Japan in 1940 is low. 73 Million would be more precise (vs. 132M for the U.S.) Additionally, Japan controlled Korea and Taiwan as well as large swaths of Manchuria. These territories just about made up the difference in population.

However, the economy of Japan and it's territories was very agrarian in character in 1940. 50% of it's domestic workers where employed in the agricultural sector in 1940, compared to 22% for the US. The US also had far more invested into mechanization in the agricultural sector. In terms of industrial war potential, population in the agricultural sector tends to generate a lower per capital war potential. So this factor itself was substantial.

Then we have the amount of capital invested in the industrial sector. Capital can be thought of as the amount of factories or industrial machines per worker. By 1940, the U.S. had been a world leader in the level of investment and automation per worker for decades. While measured in money, this can be thought of in concrete and human terms as well. The U.S. industrial sector possessed a lot of machines and a lot of machine tools, machines used to make other machines. These required the support of many experienced operators, assembly line workers and experts who knew how to set up assembly lines and machinists who knew how to create the machines used on assembly lines. This infrastructure was not easy for another country to recreate in peacetime, let alone wartime.

Additionally, the U.S. had a huge land area providing access to resources, as well as trade within a huge geographic sphere. Access to Oil, Iron, Coal, Bauxite(for aluminum) and trace metals were far easier within the U.S. Then the U.S. Also had the ability to trade in South America, Africa, and portions of Asia and Oceania. Just about the only resource that the U.S. had difficulty accessing was rubber. Japan's geographic area was much lower, and it's sphere of trade was even lower still due to poor relations with neighbors and colonial powers. The lack of resources, particularly petroleum, was a huge constraint on the Japanese wartime and peacetime economy and a driver of conflict that sparked the war. In the 1950s and later where free trade was more prevalent, this did not constrain Japanese economic growth, but in the protectionist 1920s and 30s this was a huge factor.

Lastly, Japan conscripted huge numbers of men in peace and war. Since the end of the Russo-Japanese war in 1905, Japan conscripted about 20% of the available 20 year old male cohort into the army alone. The U.S. Military had no peacetime conscription until 1940. Basically men conscripted into the army and navy were deadweight losses on the economy. Even in wartime, the U.S. was careful to avoid hampering industrial production by excessive conscription. Whereas Japan with it's overseas empire and militarized neighbors (China and the Soviet Union) was forced to keep a large army in peacetime. The war in China since 1937 was also a huge drain on the Japanese economy.

Basically Japan and it's empire were industrializing countries and not fully industrialized by 1940. Whereas the United States was the worldwide leader in industry with the highest productivity per capita.