If all the countries in the world swapped positions based on population, then…
Australia’s 22.5 million inhabitants would move to Spain, the world’s 51st largest country. This would probably be the furthest migration, as both countries are almost exactly antipodean to each other.
The UK migrates from its strategically advantageous island position off Europe’s western edge to a place smack in the middle of the Sahara desert. No longer splendidly isolated, it will have to share the neighbourhood with such upstarts as Mexico, Myanmar, Thailand and - good heavens - Iran. Back home, its sceptered isles are taken over by the Tunisians.
The US would not have to swap its population with another country. With 310 million inhabitants, it is the third most populous nation in the world. And with an area of just over 3.7 million mi² (slightly more than 9.6 million km²), it is also the world’s third largest country.