I have to assume that both their governmental and societal infrastructure was severely damaged during the war, so what are some of the reasons for their rapid growth in the post-war era?
Who was involved in the rebuilding of their respective countries, and why was it so successful?
According to economist Mancur Olson societies stagnate as elites gain control over the state and use it to enrich themselves.
Since Germany and Japan had their power elites wiped away, their economies could restart without any such dead weight.
The Rise and Decline of Nations: Economic Growth, Stagflation, and Social Rigidities,