Scott Hume, editor of BurgerBusiness.com, notes that imitation is common in the fast-food industry.
"McDonald's clones make sense if they're popular in the marketplace," Hume said. He noted that Burger King has also rolled out more differentiated products, such as a pulled pork sandwich.
The changes aren't yet making a big splash with customers. In the latest quarter, Burger King said sales slipped 0.3 percent at North American locations open at least a year. McDonald's said the figure edged up 0.7 percent.
(Read more: Did climate change kill McDonald's Dollar Menu?)
Burger King Worldwide Inc. has had numerous owners over the years. 3G Capital, an investment firm run by Brazilian billionaires, bought the chain and took it private in 2010. It took it public again last year in a deal that allowed it to earn back what it paid while still maintaining a majority stake.
Since then, 3G has moved CEO Bernardo Hees on to another assignment over in Pittsburgh, where he's heading up H.J. Heinz. 3G bought the ketchup maker with Warren Buffett's Berkshire Hathaway. In Hees' place, 3G installed another one its principals, Daniel Schwartz, who was 32 at the time.
(Read more: Burger King 'Satisfries' customers with new fries)
As for the new "Big King," Burger King's website lists it at 510 calories. The Big Mac is listed at 550 calories.
Burger King also recently introduced lower-calorie fries that it dubbed as "Satisfries." Notably, Burger King has been touting the fries as having 30 percent fewer calories than regular fries. But the comparison is to McDonald's fries, not its own fries.
Satisfries have 20 percent fewer calories than Burger King's own fries.
—By The Associated Press