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The entire cast of "Pain & Gain" went through some pretty hefty transformations to play body builders in the new flick ... and, according to them, they did it without steroids!

Mark Wahlberg reportedly added 40 pounds of muscle to his body in just two months after shooting "Ted," going from 165 to 205. He tells he did it by eating ten high-calorie meals a day, hitting the gym like crazy and taking supplements.

As for his hulking costar, Dwayne "The Rock" Johnson, we don't know how much he specifically put on, but he definitely looks more buff than ever in the new movie ... which he filmed while also training for WrestleMania.

But expect to see a leaner Rock soon -- he had to trim down a bit to star in the upcoming "Hercules," something he says is "the biggest prep physically and diet I've ever done" for a role.

So ... between the two, who'd win in a fight?

"Boxing, I'd bust him up ... wrestling, I'd get my head torn off," Wahlberg told toofab at the film's premiere. "He doesn't know what to do when somebody puts gloves on him!"

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The two headlining stars aren't the only ones who transformed their bodies.

Bar Paly says she gained 20 to play a stripper, while Anthony Mackie (below left) was in the gym just as much as Mark.

See more stars who went through dramatic weight changes for a role in the gallery above.

"Pain & Gain" hits theaters Friday.

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