Eating animals requires that they are fed. When animals eat grains, etc, they are using resources, land etc, require chemicals to treat the feed, etc and use incredible amounts of energy to sustain the animal. Also, as you can imagine, there is waste from the animal while it is living, methane gases and other wonderful things that come from the animal. Basically, it takes 100 x more energy to eat a steak than it does to eat a salad, or some beans or other veggies.
Good question that I didnt answer completely, but you get the idea. I'm sure someone else will give you more accurate information.