Matt Damon on The Informant set
Andrew Shawaf/Pacific Coast News
's wife Luciana may be pregnant
, but it's his
expanding waistline that's making all the headlines in Hollywood.
"I think I lost my [Sexiest Man Alive] title," the actor joked to PEOPLE Wednesday at the Ante Up For Africa poker event in Las Vegas.
But that's no sympathy bump. Damon estimates that he gained between 20 and 30 pounds for his upcoming thriller, The Informant
, directed by Steven Soderbergh.
"It wasn't necessarily that I needed to be fat," he said. "It was that I needed to be 'doughy.' "
Even though the weight-gain was required for a role, that doesn't stop Damon's friends from mocking him mercilessly.
"Some things are just self-evident and don't even require making fun of," his B.F.F. Ben Affleck
said at the Rio Hotel and Casino event, which raised money and awareness about the genocide in Darfur. "I mean, the man buys two seats on an airplane!"
So how did the Oscar-winner pack on the pounds?
"I just stopped working out and basically just ate whatever I wanted," he said. "I ate a lot of In-N-Out, a lot of burgers, a lot of beer and basically had a great time ... When you're in your 20s you can do that kind of stuff. When you're in your 30s, its a whole different ballgame."
Read more about what Damon eats to maintain his transformation in the Bodywatch section of this week's PEOPLE, on newsstands Friday.