The Japanese were brutal to defeated soldiers, in the same way they've always been, even during various Japanese civil wars. Unlike the Germans, however, they did not systematically exterminate, like vermin, 12m civilians who weren't fighting them. The Bataan atrocity occurred in large part because Japan was a poor country (1/9 the US GDP per capita, the rough equivalent of Nigeria doing a Pearl Harbor today) punching above its weight militarily. They had a choice between starvation rations for all (their soldiers and POW's) and feeding their soldiers adequately while starving the POW's. They chose to feed their soldiers.
The Germans never had a problem feeding POW's because they were a wealthy nation roughly at parity with the US on a GDP per capita basis. (This was how Germany managed to kill 300K Americans compared to the 100K killed by the Japanese). And yet the Germans felt compelled - on a whim - to exterminate 12 million civilians who were not fighting them or even associated with resistance movements.
The gulf between our respective cultures was reflected by the widely varied attitude toward retreat and surrender.
To Americans, retreat was an acceptable military tactic. To the Japanese, retreat was dishonorable and revealing of an inferior culture.
To Americans, surrender was what you did when you found yourself trapped and without any means of further resistance. To the Japanese, a foe that surrendered was without honor and unworthy of respect. POWs were not treated well because they didn't deserve being treated well.
I think you figures are a little off there. Japan's entire GDP was 1/3 of USA.