Theres The Beef

Before I went to sleep this morning after drinking a glass of milk a question came to mind, "How do cows become so fat?" All cows do is eat grass and drink milk. Milk which is used to feed infants before they're able to digest 'real' food. Cows milk only contains 3.6% fat. Guinea pigs I can understand with their 46%. What really baffles me about how large these animals are is the fact that they even throw up their food, re-chew it, re-swallow it; so its super digested and another reason for them to not be so big. While cows are born at around fifty to a hundred pounds I really don't see a reason for them to grow so large, what with their diet and everything. I guess cows are just 'big-boned'. Another mystery goes unsolved.