When a cat’s tail gets big and puffy, it usually means that your cat is being either aggressive or defensive, such as a cat fight. Your cat’s tail gets big and puffy to make himself look bigger. Cats do this so they appear to be larger and scarier than they actually are!
Basically, the fur all stands up, making the tail look 2-3 times thicker than it actually is. The fur on your cat’s back will probably stand up as well, making it look like your cat has a mohawk!
You’re probably thinking, why would your cat want to look larger and scarier? Well, if your cat is being aggressive, they may wish to scare another cat by appearing to be larger.
Or, if your cat is being defensive, your cat may use this tactic as a defense mechanism.
Kittens often puff up their tail when they are playing Adult cats will do this too sometimes, either when they are playing or if they get spooked!
Cats will often turn their bodies sideways when they puff up their tails. They do this for the same reason – to appear larger!
Humans generally find this very funny and do not feel that their cat looks scarier whatsoever!
Learn more about Your Cat’s Behavior from Makati Dog and Cat Hospital
Thanks: http://whydoesmycat.com/articles/74