It's probably not the only cause, but more wealth for the poor people means people more apt to contribute to the society.

Compare one billionaire buying a mega yacht and 10,000 people buying their first car, in both cases you've created say $200M in direct GDP, but in the later case you have 10,000 people more mobile to get a job or consume other goods or services.

Personally I wouldn't be surprised if that was the case. I strongly suspect less inequality is positively correlated with a host of beneficial outcomes (more access to education and healthcare, better quality if life etc.) which can in turn lead to a virtuous cycle that would in turn increase wealth.

I think the wealth increase is more easily explained by rebuilding everything we just finished blowing up, exploiting all the technological gains we made while getting better at blowing things up and using the disruption/reset as a means to achieve greater local economic maximums (e.g. historical rent seekers kicked out and things are free to become more efficient and mutually beneficial in the time it takes new ones to establish).