How Big were Cities in the Ancient World?

It is a deep seated idea among some people that things were better in the past, and that as time goes on we are heading towards more and deeper problems and are inevitably going to suffer further decay and decline.  This was particularly noticeable in the Middle Ages - though if you read Tolkien you can find the same notion alive and well in the Twentieth Century.   But it looks like the men of the Middle Ages had good cause to regard their own time as a pale reflection of what went before.

Over on Historium I have just come across a really interesting analysis of how big cities were and how much trade there was in the ancient world.  It is pretty interesting to compare with the more formal history of the empire given in Gibbon's Decline and Fall of the Roman Empire.  In particular it looks like urbanisation and trade were at their highest levels during the period of the five good emperors. Things never really got back to those levels during the rest of the empire and continued to decline even further after the fall of the empire in the West.  The most remarkable statistics are those of lead pollution.  These didn't get back to Roman levels until the end of the seventeenth century.