Seaquen, which the heroes reach in the 3rd adventure, has a population of a few thousand. Bear in my that I have no familiarity at all with World's Largest City, but if you're looking for a big city in the setting that it could represent, the best match would be Ragos, the capital of the Ragesian Empire. Or you could use it for the 9th adventure, The Festival of Dreams, which returns to Gate Pass when it is full in the grip of street-to-street warfare.
However, my (vague) understanding is that WLC is sort of a campaign setting in its own right. It might be a waste to try to shoehorn it into another campaign. Sorry I can't be of more assistance.