Of course, one way to answer this is to make a list of all of the biggest cities in the Americas. These are clearly the places that are the most populated. We could simply make a list saying that Sao Paulo in Brazil is the largest city, followed by Mexico City, New York City, Bogota, Colombia, and Lima, Peru. However, this is not a very useful way of putting things. Instead, it is probably better to say that there are at least three factors that determine where there are a lot of people in the Americas.
First, many of the most populated areas in the Americas are those where there is good access to water transportation. New York City is like this. So are Rio de Janeiro, Buenos Aires, Lima, and Los Angeles. Water transportation was a very important thing in the days before railroads and highways and it remains important today.
Second, there is industry. Many heavily populated areas are very industrialized. Many people are needed to work in the factories and therefore these areas become heavily populated.
Finally, there is political power. In many countries, there is much to be gained by living around the capital of the central government. This is particularly true in countries where the government plays a much greater role in the economy than it does in the United States or Canada. This is one reason why places like Bogota and Mexico City are heavily populated.