Japan had about 70 million people in WW2, so they had the manpower but they coudn't equip them. Britain also lacked resources, they only grew about half of their food ration, had little oil and other minerals but triple the industrial base of Japan and enough coal and iron to feed this industry.
If Japan had Britain's coal, iron and industry then they would be three times as powerful as they were IOTL. Presumably they could build a military of similar size to Britain's but with different priorities etc. For example they may not build fifteen thousand heavy bombers, instead building landing craft or armoured units. Such power could not be defeated by the resources that the US and Allies devoted to the Pacific, there would have to be a serious increase.