Minecraft wiki entry for Huge_fungus_generation, associated text: Unlike player grown huge fungi, naturally generated huge fungi have a 1/10 chance of growing with a 3x3 plus shape of stem blocks with a distribution of air and stem blocks in the corner of the plus shape, rather than the 1x1 column of stem blocks that will always generate with player grown variants. This increased trunk width does not increase the thickness of the surrounding wart block layers, but it does allow for a ring value of 4 which means that the bounding box of the leaves expands by another block.