The thing is that all city-states are not self-sufficient. They need economic inputs. To pay for this, they need economic outputs. Hence they need high productivity per human unit. This means more adults, and more productive adults, per square kilometre.
Hence, like Monaco, population may top out at 15,000 per sq km (now it's only 7,500 per sq km in Atlantis). It will have, like Monaco, perhaps a 'native' population of only 20%. But that's okay, because many of the immigrants may choose to naturalize, or just spend a lot of money here and go off into the sunset.
This will leave Atlantis a staid old folks' home coupled with frenetic cultural activity (if you include, like Monaco, casinos and le Grand Prix). It is the only sustainable fate of the modern independent city-state. If you want more, you either have to surrender some more of your independence (oh come on, there isn't a truly independent state nowadays anymore), or you have to go conquer somebody larger (yeah, as if that's a real option).
There, I've said it. And I'm a fourth-generation Atlantean.