With 7 millions of inhabitants Hanoi is not a very populated city. Not yet. 

However, the migrations converge to the center of the capital. If we look at the density, the things change: its nine central districts have an average population density of 20,000 people/km2 (London: 11 000/km2)

Therefore, the population is forced to live in very cramped conditions. 

Those photographs, taken in narrow alley and streets, illustrate the feeling of compression in those areas.