I actually had the same situation...Both cats are the same age, but Katie is much bigger than Belle. However, Belle has a stronger personality and is certainly the aggressive one. We got them both as small kittens (Belle was the runt, fit in the palm of my hand) and it has always been that way, even since they were both spayed. I think it has a lot to do with personality; Belle is not afraid of anything (except the vacuum cleaner...lol).