One of the reasons why Imperial Japan was eventually defeated was precisely because their economic capacity was a tiny fraction of the US’ even before the US economy ramped itself into production.
And while Japan was far more developed and technologically advanced vis a vis its neighbors, a lot of that came from exploiting resources from their colonies and an economic system which prioritized the aristocracy/wealthy and especially military elite over everyone else. It’s a reason why even in the 1930’s, the militarist government had no issues imposing a budget which heavily prioritized military spending above all else*.
- The buildup of the IJN fleet alone took up ~32% of Japan's entire GDP in this period.
Both Koreas were exceedingly poor due to 40+ years of Japanese colonialism, racially based oppression, maximal exploitation of its resources, and massive war related damage by the time WWII ended.
The only reason why North Korea inflicted massive casualties was that it was amply supplied with military aid by the Soviet Union and later on, both the Soviet Union and Mainland China contributed resources and personnel to help North Korea.
In contrast, South Korea got diddly from the US/Allies until after the invasion from NK commenced in 1950.
One other thing many…especially Americans forget is that up until the early '60s, North Korea had a much stronger economy and was wealthier than South Korea because it had the benefit of taking over an area which was much more developed industrially and had the lion’s share of wealth/natural resources whereas South Korea was effectively an economic backwater with most of its economy centered on agriculture and a lack of natural resources.
This aspect if understood only highlights how much more impressive South Korea’s economic rise to the point of not only overtaking its Northern counterpart despite its great initial advantages, but also becoming economic peers of current advanced industrialized nations in only a few decades. It also underscores how badly NK’s Kim dynasty squandered North Korea’s economic/resource advantages it had over South Korea so that it fell behind starting in the mid-60’s onward.