Why does my kitten puff up when she sees herself in the mirror?

Your kitten is likely not puffing up when she sees herself in the mirror. Instead, she is making herself look bigger in case the reflection she sees is actually a threat. Some cats may even hiss or growl at their reflection in the mirror, which is a display of aggression because they think there's an intruder. Some kittens don't mind their reflection and instead may think that the reflection is a potential playmate.