Most "smaller animals" do not exhale methane the way cattle do. It is a byproduct of the digestive system in animals with two stomachs for the most part.

Pigs also digest feeds that produce methane.

Humans on veggie diets also produce it. If one were to connect the dots, we all may need a carbon tax stamp to eat broccoli or chili with beans.

Think of the constitutional arguments had the Founders included farting within the bill of rights.