Chickens have changed. Today's broiler chickens are several times larger than broiler chickens of past decades -- and a new study by researchers in Canada offers an explanation for why the birds got so big.

For the study, the scientists raised three breeds of broiler chickens : one breed that was common in 1957, another from 1978, and a third from 2005, called the Ross 308 breed, CBC News reported.

"We fed them exactly the same things, so we did not provide hormones," lead author Dr. Martin Zuidhof, associate professor of agricultural science at the University of Alberta, told the CBC. "The only difference that was part of our study treatments was the genetics."

(The U.S. Food and Drug Administration has long banned the use of hormones in poultry production .)

What did the researchers find? The Ross 308 chickens grew to be much bigger than the 1978 breed and four times larger than the 1957 breed .

"We had never actually tested our 1978 line before, but where they fell were very consistent with what we believed would be the case based on historical selection for growth rate and efficiency ," Zuidhof told the Canadian news channel CTV.

In other words, today's chickens are bigger simply because they were bred to be bigger. Should we be concerned about eating these big birds?

"There is no danger in eating larger chickens," Zuidhof told The Huffington Post in an email. "That would be comparable to saying it is more dangerous to eat bigger carrots because theyre bigger."

The study was published online in the journal Poultry Science on September 26, 2014.