I'd question the assumption that North America had more settlers than South America.

Do you mean nowadays or in the 1600s?

In the beginning Portugal and Spain "created" colonies along Africa and America for commercial profit only. They would negotiate with locals and not create any settlements. The goal was simple, gain access to new trade routes to Asia and make profit from the product trade between European and Asia.

I do not know exactly when, but as the time passed, the new discoveries and with the trade routes evolution there were two main areas.

• Africa - Lots of tribes to negotiate, natural resources and slaves that were sold by rival tribes.
• South America - Lots of land to agriculture exploration but lack of workers. Meaning the colonization of South America had to be done with people from Europe and Africa. I remember learning that most of the Portuguese that started the colonization of Brazil were single men wanting to be rich.

In North America the colonization was done by English families with the goal of Agriculture exploration needing workers. Also there were only a few tribes, that did not control as much land as the tribes in Africa.

As centuries passed, in the 1800s, locations in Africa and South America needed hard work in plantations, the owners of these properties wanted slaves, not settlers. North America had space to grow and cities growing, cities where settlers could do the same work as they did in their home towns.

As a comparison you can also see the region occupied by California, Nevada, Utah and Arizona. The most extreme climates do not have big cities, but places that were seen as trading points (Coastal cities and cities along terrain routes) are nowadays big cities.