Humans are constantly figuring out how to get the most out of the same amount of space. Crop rotation in the middle ages was the first real innovation in this (as far as I know). Currently, its being done through hormones. We give animals (and plants) hormones to make them grow up quickly and develop the parts we like to eat to extreme proportions. They are given the minimum of space required to survive. This means more product on the same amount of space. Because they grow quickly, they can be slaughtered quickly and quickly replaced.