In reference to this and your earlier post:
It is true that Japan had to maintain a very large empire during the Second World War, and they were certainly overextended. However, you must realize that much of their influence is over ocean. Although Russia is on par with the size of Indonesia, Japan doesn't need to have troops stationed over every square mile of that, like Germany did, because they are a major naval power. Furthermore, although the size is similar, the actual square mileage of landmass will not be.
It was the massive losses inflicted on the Japanese navy in 1942 that began the downfall of their empire, particularly the battles of midway, the coral sea, and Guadalcanal; all of which were grapples for air, land, and naval supremacy.
If Japan could no longer keep naval superiority, they could not defend their spread out empire, and the rest is history