Map of the world rearranged by population NOV 23 2010
If all the countries in the world swapped geographic positions based on population, then you'd have something that looked a bit like this:
Take the world's largest country: Russia. It would be taken over by its Asian neighbour and rival China, the country with the world's largest population. Overcrowded China would not just occupy underpopulated Siberia - a long-time Russian fear - but also fan out all the way across the Urals to Russia's westernmost borders. China would thus become a major European power. Russia itself would be relegated to Kazakhstan, which still is the largest landlocked country in the world, but with few hopes of a role on the world stage commensurate with Russia's clout, which in no small part derives from its sheer size.
Canada, the world's second-largest country, would be transformed into an Arctic, or at least quite chilly version of India, the country with the world's second-largest population. The country would no longer be a thinly populated northern afterthought of the US. The billion Indians north of the Great Lakes would make Canada a very distinct, very powerful global player.
The full map is here. Interestingly, four countries stay in the same positions: the US, Ireland, Yemen, and Brazil.