Arisaema triphyllum is highly variable. I have yet to find a citation for the range of plant sizes, but there are many described varieties. I've seen a few in VT that were close to 1 meter tall! This plant is interesting because I see both the large specimens and some much smaller ones in the photograph. So, ecological variation such as hydrology or sunlight don't seem to be playing a huge role in the determination of the plant size, otherwise all the plants in the photograph would be roughly the same size. Maybe it's genetic.