Whats with movies and tv doing this?

Like in victorious Andre says about cat "hey she's only 90 pounds, how much pee can she hold." shes obviously more than 90 pounds. I'm 100 pounds and clearly a lot thinner than her and so are lots of other people. It's done in lots of movie like girls who are clearly 100+ pounds said to be 80 or 90. Why is this so? It's just weird. Please and thanks.