What does they mean by "largest" ?
Because "it'll be a generic city, usable by all levels of gamers."
And well, in one campaign world, metropolises may be commonplace, while in another it'll be a rarity. In one campaign world, you'd have to have a population in the billion to have a serious claim at being the world's largest city, while in another, the DM would never be able to fit a city with a population of more than 500 000 without destroying the world's consistency.
And then you've got the problem of not being able to use the same description for a city that can be the home of anywhere from 50 000 to 5 000 000 000 people. :\