Except Waterdeep is modeled after Europe more. And lacks the basics that allowed those non European cities to hit those sizes.
Even magic only goes so far. It's not producing the food eg plant growth but transporting it.
FR doesn't have a lightning rail network, magical steamship en masse, magical food replicator as standard tech, magical trucks imperial capital, or fertile floodplain with year round growing season (or two crops high calorie crop like rice).
Because magic only goes so far.
5E maps are roughly same size as 3.5 but if the 2 million figure is accurate it's increased in size by a factor of 15-20.
Dessarin valley and area around Waterdeep is plausible enough at 2 million.