Steven, you need to know that the mushroom is not the organism, it is the fruiting body (the same way an apple is not the organism, the organism is a tree). A 600 pound mushroom hints at an inconceivably large mycelium. Indeed these can extend for miles (and be thousands of years old, putting them among both the oldest and largest organisms on earth). So yes, there are a small number of organisms bigger than the biggest mushroom, but the mushroom is not what we’re comparing them to.