I just did what you suggested and looked at Kinshasa and it looks HUGE to me. Endless rows of streets. Reminded me of Mexico City (and I just compared satellite imagery and they seem similar in that respect - and they are similar in terms of population). Big cities with few skyscrapers and tons of low rise buildings are quite common. In the west, Paris is an example of a populous city without many tall buildings.
Kinshasa's urban area (not the entire province) is generously estimated at 600 square km.
Mexico City is 1485 square km.
Mexico City's population, within that 1485 km^2 envelope, is most commonly given as ~9.5M. (It's ~20M with satellite towns and regions.)
And then consider that most of Kinshasa is comprised of buildings that are very low to the ground -- far lower than in Paris -- whereas Mexico City is in places very dense.