This looks very Eurocentric to me. China and the various Indian empires are getting short-shrift here, at least from an economic and military power perspective. It's a cool diagram, but what was the methodology used to generate this?
EDIT: Makes sense in the context of the times. Would be interesting to see an updated version, based on modern historical estimates.
No idea in the case of this map, but in Why the West Rules--For Now, the author comes up with a fairly complex social development index which factors in things like population, largest city, agriculture, etc. http://shc.stanford.edu/news/research/why-west-rules%E2%80%9...
He focuses solely on the Western and Eastern cores however.
If you're interested in the topic, you might want to check out that book. (It's a good read as a whole as well.)
It has the advantage of being a little more modern/complete than the original Histomap, even if it's a little lighter on that nice minimalist aesthetic. I have one hanging on my wall and it looks good.
EDIT: Makes sense in the context of the times. Would be interesting to see an updated version, based on modern historical estimates.