From the (very limited) stuff I've seen on this issue, I believe the environmental burden is smaller for smaller animals. So sheep and chickens are more defensible than cows and horses, say.

There was a period of time (before converting to vegetarianism) where I tried to mostly eat chicken and other smaller animals, for that reason. But the more I learned, the more my best guesses about how animals experience suffering led me to believe that while chickens are more environmentally friendly, they also suffer more, and I wasn't sure how those factors weighed against each other.

Obviously I solved the issue eventually by becoming vegetarian. But if animal cruelty isn't a concern for whatever reason, it does make more sense to eat chicken than cow. (Not entirely sure, but I think the ratio of energy input : output when farming cows was something like 40:1, whereas with chickens it's something like 7:1)