Tiny/Huge Island is the 3rd secret level in the Mushroom Kingdom. It's based on one of the later levels of Giant Land from Super Mario Bros. 3. The main difference between this level and others is that all the enemies are much larger than usual, about the same size as the player. Despite their large size, they can be dealt with as easily as when they were normal-sized. You can enter certain doors in the level to enter a "tiny" version of the level, where all the enemies are normal-sized.
This level was previously a recreation of the first level from Giant Land in Super Mario Bros. 3 before it was reformatted to make it more original.