In the modern world, a city with over a million people in it is nothing special. There are Chinese cities with more people than the whole of Portugal.
But in the medieval world, or a fantasy with a vaguely realistic approach to demographics, things were very different.
For a start, the rural population was much larger than the urban population.
Villages could be spread over a significant distance, or be a very simple small settlement which would basically have a few houses, a single street and a parish church. Around 150 people or fewer would probably live in a village, but obviously that varied. It would not be unusual for everyone to know everyone in a village.
A town was a bigger deal, and had one key attribute: the market. The market meant that traders (even if just occasional traders, such as subsistence farmers selling the surplus from a bumper crop) would travel to the town and do their business. This was advantageous all round. Traders got to earn cash, the local lords got to charge tolls to use the roads, bridges and trade within the town. Towns were the beating heart of the economy, but weren’t necessarily all that large. Several hundred people, perhaps, but that would include craftsmen that would not be found in villages. Towns could be home to thousands rather than hundreds of people (although you could argue at that point the difference between a town and city was almost academic).
Cities, in England at least, were defined as having a cathedral (and, therefore, a bishop). Economies of scale meant that cities would be richer, man-for-man, than other, smaller settlements. However, so many people crammed together almost made hygiene and crime more troublesome. Not to mention that fire could absolute devastate a city. A city might only have a couple of thousand people. Over ten thousand would be very significant in, say, the 14th century. Only a few were ever larger (Byzantium was enormous during its height, as Rome had been earlier). A city of one hundred thousand could well be the seat of a continental empire. According to Ian Mortimer’s The Time Traveller’s Guide to Medieval England (which I heartily recommend and review here), London had a population of just over 40,000 around this period.
I used to have links to a number of fantastic medieval demographic calculators, but sadly they seem to have become defunct.
It’s also worth pointing out that populations were more vulnerable at this point in history than today, and compared to the past (I’d rather fall sick in ancient Rome than medieval England). Disease was generally not handled well, with cures often useless at best and harmful at worst. Infection was not well understood, and in the middle of the 14th century the Black Death swept through England, killing a very significant proportion of the population (so much so that the price of things like swords declined, because so many sword-owners dropped dead, and food rose, because there were fewer peasants to work the land).
Fires, as mentioned above, could rip through medieval settlements, which often had wooden houses packed very close together. Not to mention the perpetual state of warfare that existed during the 14th century.
Nowadays the population, globally, only goes one way, but back then populations rose and fall as prosperity and advances in farm and mill equipment were balanced out by disease and war.