FWIW, if anyone is interested, this is the size comparison for the ancient cities, all at the same scale:
Populations (estimated) near their "peaks":
Athens: 350-500,000 (includes women, children, and slaves--who could number as much as 80% of the population!)
Constantinople: 200-800,000 (in varies widely over the centuries, often about 400-500,000)
London: 50-80,000 (depending on era)
Paris: 200-300,000 during the middle ages
Rome: 1,000,000 (reported as high as 4-5 million, but reports contradict on just who was counted: men only, citizens only, females and children, even slaves...?)
York: 10-15,000
Obviously, this isn't exact, but it can give you an idea of how crowded much of these cities were. Rome, for example, would starve without the wheat crops brought in, and Paris has a history of famine due to loss of crops. I think it is important to note this largest cities in ancient to medieval times could not survive without the influx of trade and food (much like our large modern cities, of course!).
Fire (due to how close buildings were), disease (due to lack of general health care in many cases, population crowding, and exposure), and famine (crop failures, etc.) were the primary fears, along with invasion by rival cities (both in the region and from abroad).