MarketWatch has an interesting article on how chickens have been bred, and altered, to be twice the size they were 60 years ago:
Nowadays, most chickens spend their whole lives in a small confinement together with thousands of others from birth to death, with no possibility to roam or even move. They are raised to reach the “slaughter weight” as fast as possible through excessive feeding and lack of exercise.
FreeFromHarm has more information regarding the role chickens, and their suffering, play in the world:
Chickens are the largest number of land animals bred specifically for human consumption. Globally, more than 40 billion chickens are slaughtered each year for meat out of an estimated 65 billion animals killed annually for this purpose. Nine billion chickens die in the United States alone each year. Approximately 5 billion egg-laying hens are in battery cages throughout the world, many of them in production complexes holding a million or more birds.
Modern chickens are bred to be so heavy that they can barely stand or walk, causing extra suffering for them to endure.
All that needless pain, suffering and misery for what? A sandwich? Fuck. That!