As we begin 2017, Americans can look back at 2016 as yet another year with record-high chicken consumption—an average of 89.6 pounds per person. That’s more than three times as much as our grandparents ate.

Chickens we eat today are twice as big as they were 60 years ago. In 1955, the average weight of chickens sold on market was 3.07 pounds, while the number for the first half of 2016 was 6.18 pounds, according to National Chicken Council, a nonprofit trade organization based in Washington, D.C.

Firstly, chicken breeds today are more cost-efficient than 60 years ago. In order to shorten production cycle and cut cost, the selective breeding for broilers — chickens raised for meat rather than eggs — prefers faster growth rate and higher feed-to-meat ratio — meaning the pounds of feed it takes to gain one pound of meat.

The time it took to grow a newly-hatched chicken for market has been cut half since the 1990s to only less than 7 weeks from 16 weeks in 1925, data from the National Chicken Council showed. And it only takes less than half feed to get the same amount of meat.

Animated GIF by Stefie Gan.

The trend started with the 1948 contest that invited farmers nationwide to develop the “Chicken of Tomorrow” with specific goals — bigger, meatier, faster growth. As a result, Arbor Acre breed, the crossbreed of the two winners, has become the grandparents of most commercial meat chicken we eat today worldwide.

There were massive genetic differences as a result of selective breeding by raising chicken breeds from different eras under exact same conditions, a 2014 study by researchers at the University of Alberta, Canada, observed. The result was stunning: At the same age, the 2005 breed had grown to about four times as heavy as the 1957 breed, despite being fed the same food.

Secondly, chicken companies have also achieved higher efficiency in their raising process — both in terms of quantity and size — through economy of scale and evolved poultry science.

“Chicken was incredibly expensive and largely considered a luxury good,” said Emelyn Rude, author of the 2016 book “Tastes Like Chicken: A History of America’s Favorite Bird,” “In the 1950s and before, chickens were only sold in whole form, which entailed an incredibly labor intensive process of butchering and processing before they could be cooked and eaten at home.”

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. Although hormones and steroids in the poultry industry are prohibited by the Food and Drug Administration, antibiotics are regularly used by farms as growth agents.

“Scientists know more about chicken nutrition requirements than any other creature on the planet, and so advanced chicken feed helps contribute to their tremendous growth rate,” said Rude.

Thirdly, the American diet has experienced a dramatic shift toward processed food. As more pre-cooked and convenience products became available, bland, cheap and healthy chicken became the perfect item to feature in these type of meals, and it made financial sense for manufacturers to raise bigger chickens for TV dinners and other prepared foods. “Most cookbooks starting in the 1970s only call for boneless, skinless chicken breast, a product that was virtually non-existent before the advent of chicken processing,” Rude told MarketWatch.