As promised in this BPotD entry for the Asian Garden, here is another photograph of Petasites japonicus var. giganteus, or Japanese butterbur. As alluded to by Weekend Gardener in the written accompaniment to his submission, this plant can be an aggressive grower given the right conditions. I'm hesitant to attach the moniker “invasive” to it (like so many others have on the web), primarily because its ability to disperse is limited. That being said, it may indeed be biologically invasive in some areas, so caution should be exercised if you are considering growing it.
The size and structure of the leaves hint at the habitat ecology of the plant – moist soils in a shady forest. Large leaf surface area typically equates with a high rate of water loss, hence the need for moist soils. The large leaf surface area is also a mechanism for capturing as much available light as possible, a strategy typical of growing in shady conditions. The leaf itself is relatively thin compared with the sturdy similarly-sized leaves of some Gunnera. When considered in tandem with the surface area of the leaf, thin large leaves imply the plant cannot be exposed to high winds. Accordingly, forests offer protection from any potential mechanical damage or dessication caused by air movement.
Small note on the taxonomy – some excellent references suggest variety giganteus, while others use subspecies giganteus. I've used variety, but if anyone wants to convince me otherwise, I'd definitely listen to any argument.