I am specifically curious about how the United States managed to so grow so large so quickly in terms of population.
I think you are overrating the population of the US :
And for total population :
So as you can see, the US did not really "grow large" compared to the rest of the world. It has pretty much the same density as Africa, and is 3 times less dense than Europe (not to mention Asia).
Source : http://data.worldbank.org/indicator/EN.POP.DNST/countries?display=default
hi! for China and India, there are a few posts in the FAQ (link on sidebar):
For the US vs Canada, here are some posts:
First of all, the US, China and India are huge countries in terms of area, so they would naturally have very large populations. However, as someone has just written here, the US actually has a very low population density: if the US had the same population density as the UK it would have around 2.5 billion people.
In general, the regions of the world with the highest population densities are those that can support large populations, ie places with very fertile and productive soils, a hospitable climate with ample rain, and relatively flat terrain. The regions of the world that fit all these criteria are Northwestern Europe, the East Coast of China and the Gangetic Plain.
To answer your question of why the US population got so large so quickly, I'd say a combination of 400 years of mass immigration and high fertility. Here's an article that you might find interesting (if the Mods don't get pissed off with me for putting external links here): en.wikipedia.org/wiki/Demographic_history_of_the_United_States