Nomura's jellyfish is one of the larger species of jellyfish with an overall width that can span up to 6 feet and 7 inches (or up to 2 meters), larger than the average human height. This massive jellyfish is not all fluff either, weighing in at a hefty 450 pounds (220 kilograms). Their large size accounts for some of the damage that can be seen when large swarms of jellyfish approach a coast, particularly when you have thousands of them in a single area.
The jellyfish are keen survivalists, they will utilize most of their energy towards growth, but once under attack they release hundreds of gametes into the water where they fertilize and then attach to various underwater structures to await maturity where they will create yet another jellyfish swarm. So for everyone that you attack and manage to kill, hundreds could take its place.
This survival method of the species has caused quite a headache for Japanese officials and local fishermen who also have to deal with hundreds of dead fish in the area. This creates an additional problem for the locals and the fishing industry. Due to the high toxicity level of the jellyfish's venom, even if the dead fish are recovered immediately, once stung they are no longer safe for human consumption.
These large swarms are becoming more and more frequent for Japan due to a couple of reasons. Global warming is providing warmer sea temperatures which are favorable for Nomura's jellyfish. Additionally, with the heavy amounts of overfishing, hunting, pollution, lost of habitat, and other human activities decreasing the number of natural predators, the jellyfish are left unchecked. Japanese industries have used this increase to exploit an industry of jellyfish based products, using them in everything from ice cream and tofu to skincare and are developing various techniques to attempt to capture the jellyfish; however, given their strong survival extincts, capturing, harassing, or killing the jellyfish will only make the problem worse.
Checkout this footage to get an idea of just how big the Nomura's jellyfish is.