Minecraft wiki entry for Huge_fungus_generation, associated text: The ring value (l) at a specific height (k) is equal to 2 if k < h – a random integer up to 3, otherwise it is equal to 1, also if i > 8 and k < j + 4 then l = 3. This statement allows for 1 3-wide layer of a huge fungus to form, this can only ever occur in tree’s that are at least 8 blocks off the ground. While it may look like some huge fungi have more than 1 layer of 3 wide growth, these additional layers are actually vines which are generated separately from the ‘brim’ (3 wide layer).