Starting with Fat Cat, I wanted him to be tall as well as massive in proportion in order to give him a sense of a scale, superiority and higher authority. I also wanted to give him an intimidating aura; naturally, us humans see larger creatures as intimidating and scary.
With Issac, being a sidekick as well as mode, I saw it fit to give him a stout appearance to enhance his comedic effect. Since he's a super-genius, it would be fitting to apply the phrase: "more brain than brawn" to him.
Kat, since she is the main heroine, would have to be well proportioned and heroically built, so I gave her an athletic build, with a tall, but still average height.