Mark Wahlberg is sighted on the movie set of 'Pain and Gain' on April 24, 2012 in Miami, Florida. Credit: Uri Schanker/FilmMagic

Marky Mark isn't getting juiced.

Mark Wahlberg has been looking seriously beefy (in acid washed jeans and parachute pants!) shooting Pain and Gain in Miami. Starring Wahlberg, 40, and Dwayne Johnson, the 90s-era flick is about two bodybuilders mixed up in an extortion and kidnapping plot.

Wahlberg's pumped-up physique in Pain and Gain -- arguably even more massive than the actor's days as rapper and underwear model Marky Mark in the 90s -- has some fans even speculating whether he's using steroids to get the look.

"Mark Wahlberg definitely is not using any kind of enhancements to pump up," a source tells Us Weekly. (Indeed, Wahlberg's rep tells Us that the star has never used steroids.)

"Even though the film is about steroid abusing gym rats, he took it as a personal challenge to get to the size he is at now and has been training non-stop," the insider explains, adding that the star has been using state-of-the art equipment on location.

His already-huge costar Dwayne "The Rock" Johnson just "inspire[s] him to work harder," the source adds.

Despite the chiseled, muscular look, Wahlberg (now a married father of four), joked with Leonardo DiCaprio that their days as the hottest men in Hollywood are over.

"I'm sitting at a table with a gentleman who is better looking than you and is in better shape than me," Wahlberg told DiCaprio, 37, during a speech at the Hollywood Foreign Press Association's luncheon in Beverly Hills earlier this year. "So, we're both out of a job, dude. Taylor Lautner! We're screwed."