Hong Kong: How a 'barren rock' became an Asian powerhouse
Population itself in Hong Kong is 56th largest in the world, followed by Amsterdam in 57th largest one.
But, if you have visited both of the cities, you won't feel that. Hong Kong would seem to be much larger than Amsterdam.
The reason is population density. Hong Kong can't expand its suburban area, because border between the city and Mainland China is just a couple of tens kilometers away from downtown.
If neighboring Guangdong Province is separated from Mainland China, Shenzhen and Dongguan would have been developed as suburban cities.
In that case, subway networks of Shenzhen and Dongguan would be different from what they are.