Density alone is not an argument for mega-cities, though. While it&#x27;s possible that the economies of scale keep increasing with size -- ie. maybe the electrical network of a dense city of 5 million people is more efficient than that of a dense city of 100k people -- there is probably a point somewhere between those two points where the gains are only marginal.<p>A dense city of a 100k can support an excellent bus network, a dense city of 250k can additionaly support an excellent networks of trams or subways. 500k gets you direct access to high speed rail and beyond that I&#x27;m not sure what additional millions get you in terms of public transport -- an airport, I guess.<p>And a dense city of 100k people is more walkable than a dense city of millions: In the former case, you can pretty much reach the whole city area by walking (if you are a healthy adult). In the latter, you can only walk in your general neighbourhood -- which probably ends up being much like a city of 100k itself, meaning you rarely have to venture outside for everyday needs (shopping, services, culture, even your job if you&#x27;re lucky).