Apples and oranges, my friend.

If you look back to Roman London in the 2nd century, the population was about 60,000. After the fall of the Roman empire, London continued to function as an Anglo-Saxon city but it didn't have the benefit of a major empire controlling and trading from it. It was a relatively busy port, but it was repeatedly attacked by Vikings. By the time of the Norman Conquest, it was much smaller than it once was, but if you look forward about 200 years, the population of London was probably around 80,000 and continued to grow.

In comparison, you can look at a city like Uruk which grew from a fairly small farming village into the center of government and society for a major civilization over about 800 years. The population grew exponentially as the city grew, because that is where the jobs and food and money were.

There were times when medieval cities were large and when Sumerian cities were small. If you want to compare both cities at the height of their prominence, Uruk had about 80,000 residents during the "Uruk Period" of Sumer when it was the most influential city. London in 1914 (I'm gonna call that the height of London's power, but feel free to dispute it) had about 4.5 million in the official city, but 7.1 in the "greater London area".

Edit: added some sources