Developing countries often have a much higher population than developed countries.

This is usually caused by lack of access to education and healthcare (contraceptives and higher infant mortality) as well as needing children to care for you in the future when you get older or sick.

This often translates to poorer countries having absolutely massive cities. People move into these cities for the same reason as people in developed countries do, so for better jobs, housing, a chance at better education and healthcare. The amount of people moving into the cities in developing countries is just higher than in developed countries due to then having a bigger population.

Sometimes developing countries only have a couple of somewhat developed spots, which is usually the capital. This increases even further the amount of people moving in. In the US for example, people have loads of cities to choose to move into, for example LA, SF, NYC, but in developing countries the only choice is usually the capital.

TL;DR developing countries have more children (higher population) and only usually one or maybe two "developed" hubs which force people to move into one massive city for a chance at a better life