Minecraft wiki entry for World_size, associated text: After a world has been created, it can be expanded to a larger size. Larger worlds cannot be reverted to a smaller size. In this process, there is an option to "Overwrite World Edges", disabled by default. The description of this option reads: "When enabled, the outer 32 tiles around the world edge are overwritten by newly generated tiles to make the world size increase seamlessly. When disabled, the world size increase will leave all original tiles intact." Starting with a smaller world can be beneficial, as one may choose to gradually expand when an update makes changes to the world generation, and could therefore generate new chunks with new features.