The obvious answer is because Wizards of the Coast wanted it that way, but within this fictional world what could have caused this massive population increase? Let’s explore.
Greater access to magic and food? If anything in earlier editions magic was even more powerful, and knowledge of making magic items (for things like creating food) was likely greater, so this doesn’t seem likely as the reason for such a large population increase. Also in 1064 DR the population was 50,000. So in about 300 years (1372 DR) it had only increased to 132,000. Plus other cities have magic and it’s not seemed to have this effect on cities previously.
Poverty? Places in the real world with significant poverty tend to have very high rates of population growth. This is because families are very large in order to have enough working members that they can survive (either through income or farm work). Waterdeep doesn’t really fit this bill though. It’s pretty much described as being the greatest city on the coast while all other cities have fallen into decline (Baldur’s Gate has become Gothem, Neverwinter had the rift, etc). I’m not saying there isn’t any poverty it just doesn’t seem like the main cause. Especially given that Waterdeep Dragon Heist states that the city is the most educated city in the world, with priests of Oghma freely educating the masses (the more educated a population is typically the lower their birth rate). I also would have expected other places with poverty to see population increases (and that hasn’t happened).
Industralization? Especially in the 1800’s through to the early 20th century this was a major driver in huge amounts of population growth in cities in western countries. But the Forgotten Realms setting has always only at most skirted the line between industry and a more high fantasy setting. You’ll get games like Baldur’s Gate showing sewers with pipes and valves in them, as well as mining carts on rails, suggesting at least some level of industrialization. But factory work and manufacturing have never really been a huge part of the economy of the Forgotten Realms. If anything a lot of the work still seems to be done by specialized producers at the local setting (smithy’s, etc). Farming as well still seems to be a pretty common enough form of work in the world for it’s people. Nothing certainly suggests this is becoming more mechanized.
Massive migration? On the surface this would seem to be the most plausible explanation. With the spellplague and all the problems that emerged over the last 120 years Waterdeep became the destination for people in this region to flee to. This certainly seems supported by the fact that Baldur’s Gate’s population has only see modest growth (maybe around 40,000) between 1358 and 1479 (I’m going to choose to ignore the 1372 figure). Luskan in that same period declined from about 16,000 to 4,000. Even Neverwinter only had a population of 23,000 in 1372 DR. Yet, none of these figures really support a growth of nearly 2 million people.
It’s really hard to feed a population this large for a city without industrialization. Even the Romans never managed to do it with Ancient Rome, with its population peaking at around 1 million – 1.5 million or so. You need a lot of ready access to fish and food for this to work and even then you run the risk of depleting your fields and running out of fish within a close enough proximity to the city (and magic to preserve it isn’t going to be available for your average commoner).
This also does not seem to be the case of it referring to the region over the city. The compendium in the recent Tyranny of Dragons rerelease clearly states that the city hosts some 2 million residents. Similarly, the rerelease states that it is 2 million residents – which to me suggests people who are there year round and not seasonally.
So this leaves what? The Goddess of fertility really working up a storm getting people frisky? Some censuses over the years that have been substancially underreporting?
I’ll be honest with you all I didn’t really have an explanation for this going in. But I promised a pointless examination here and if nothing else I’m a man of my word.
Maybe you all can think of a better in-world explanation for this change. So what are your thoughts? (other than that I have too much time on my hands to write out something like this…that is a given).