I admit, this comes from a very "american-centric" mindset, but I'm often caught off-guard when I realize just how populous other parts of the world are, outside of a) Europe, and b) India/china. I'm sure there was some 6th grade geography chapter that covered all of this, but I just haven't given it thought in the 30 years since.
I knew Indonesia had a high population...but I never really stopped to let it sink in that their population is about 83% of the US population.
If Indonesia and Nigeria were to somehow join forces in something, their population would be 50% bigger than the US.
Lagos in Nigeria has more people than Los Angeles. Cairo, Egypt has more people than New York City.
The population of Africa is more than twice the size of the population of North America.