It's true that cats and dogs have a non-negligeable impact on the environment. One study [1] estimates for the US, which has around 160m cats and dogs, the impact to be around 20-30% compared to humans.

I think it's fair to assume that globally this number is much, much lower than 20%, since no country has as many pets as the US.

1: https://journals.plos.org/plosone/article?id=10.1371/journal...