I have just discovered that in Boys Town (Nebraska, the USA) there is the reputed World's Largest Ball of Stamps*. It measures 80 cm in diameter, weighs about 270 kg, and is estimated to have over four million of cancelled stamps.
The origins of the ball date back to 1953. The Boys Town Stamp Collecting started to stick the less valuable stamps on a golf ball, layer upon layer. It must have been and exceptionally cold and boring winter. The ball reached its current size by July 1955.
*Someone claims he saw a bigger one in Topeka (Kansas, the USA). Maybe he refers to the World's Largest Ball of Twine (It seems there is some big balls obsession around the world...).