Why birds aren't as large as cows...

...it's not what you think. You would figure weight and aerodynamics, right? Guess again:

What Limits The Size Of Birds? - ScienceDaily (June 16, 2009) — Why aren't birds larger? Fifteen-kilogram swans hold the current upper size record for flying birds, although the extinct Argentavis of the Miocene Epoch in Argentina is estimated to have weighed 70 kilograms, the size of an average human. In a forthcoming article in PLoS Biology, Sievert Rohwer, and his colleagues at the Burke Museum at the University of Washington, provide evidence that maximum body size in birds is constrained by the amount of time it takes to replace the flight feathers during molt.