Here's food for thought: Some cities are considerably more densely populated than others. Imagine packing all 6.9 billion people in the world into a city you know. How much space would that megacity take up?
Per Square Mile made these infographics to give you an idea of what you'd find. In the image above, they stuffed the world's people into a city as dense as Houston. We'd take up most of the continental United States.
Now New York, with everyone fitting into Texas.
And now Paris. Kind of makes you wonder what life would be like. [Per Square Mile]