I think a more relevant question is how much natural habitat is destroyed by the factory farms that are used to make pet food. The number of farmed animals killed is kind of irrelevant when you're talking about the health of wild populations.

I don't think these numbers are comparable. To the best of my knowledge one cow can fill many cans of cat food, whilst a cat may eat many geckos in a day. Similarly I may eat an entire rabbit in a day, but it'll take a while for me to eat a sheep.