from Slate Magazine. They got the chart from Goldman Sachs who just published a report with their favorite 100 charts. This was one of them. If you go to their site here, you can click the chart and enlarge it.
You'll see, for example, that in 1000 CE, Kaifeng was the largest city in the east with a population of almost 1,000,000. And Cardoba was the largest in the West with a population of about 200,000.