Even in the absolute worst-case scenario (for Japan) Operation Downfall where Japan suffers Paraguayan War levels of death (60-70% of the population), the US would never annex Japan. Whether it's 20 million or 70 million people, annexing a country full of people who hate you and is also a smoldering ruin (in the case of the 20 million population Japan, is even more of a smoldering ruin) is a pretty terrible idea. What politicians would want that?