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Fast food giant McDonald's has quietly made a change to one its most popular items: the Quarter Pounder.

The sandwich now defies burger math and includes 4.25 ounces of beef, slightly more than its former size of 4 ounces before cooking.

No word on how much the beef shrinks after hitting the grill, but the former 4-ounce patty cooked down to 2.8 ounces after heating.

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McDonald's was not immediately available for comment about the change.

The bigger burger follows an uptick in beef prices at the chain during the last quarter.

On its most recent earnings call, the chain noted domestic commodity costs rose about 1 percent, mostly due to climbing beef prices.

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It will be up to the chain's operators to decide whether to raise prices for the bigger burgers, one source told CNBC.

Citing an internal document, CNBC first reported in late June the switch was going to occur. The company declined at the time to elaborate on the burger's expansion.

The move comes as McDonald's seeks to improve its core menu as part of an ongoing turnaround led by CEO Steve Easterbrook. The initiatives include different cooking methods, such as changing how it sears and grills its beef to deliver hotter and juicier sandwiches, and a new drive-through ordering process to improve accuracy.