Or, there's the explanation that sustaining a city's worth of persons requires more than a city's worth of land. Logistically, how could a city exist occupying the entirety of the island on which it was built?
Personally, I'm of the opinion that the grid effect is some kind of artifact from the methodology of the mapping. Even so, I don't really think that a size argument is sufficient to refute that the image we see today may somehow represent ancient ruins or something.
Seriously - what did they do? Eat only fish? Boat in food from western africa everyday?