Here's why it happens:
* Threat Display: The puffed-up tail, along with other body language like flattened ears, arched back, and hissing, serves as a visual threat display to intimidate the opponent.
* Making Themselves Look Bigger: By fluffing their fur, cats make themselves appear larger and more intimidating to potential attackers.
* Insulation: While primarily a threat display, piloerection can also help insulate the cat, especially when they are stressed or in cold environments.
So, it's not about the tail getting bigger, but about the cat strategically using its fur to communicate its intentions and appear more formidable.