Well it's important to remember that Japan was a great power level economy before that war and that while a lot of damage had been done, they had a lot of that still left. The damage done in Japan was significant, but there was a large vibrant industrial sector still left at the end of the war, and there was a skilled industrial workforce to go with it. It would have been surprising to see them not return to being a top five economy.

Additionally Paul Kennedy posits an interesting idea on how they leapt into second place in his book Rise and Fall of the Great Powers. He suggests that it was due to the lack of a need to spend on defense spending. For about a half century, Japan spent significantly less on defense than a great power traditionally would. They were placed under the US's defense umbrella, and were actively discouraged from spending money on defense. This gave them a 2-5% annual boost to their general economy and allowed them to invest in growth policies, using the money that other countries would have spent on a military. And because the US was willing to buy Japanese components and equipment, and base American military in Japan, Japan was able to gain a lot of the economic bonuses of military spending, without having to have the accompanying disadvantage of military taxes. Over the decades a small 2-3% advantage in growth rates compounds.