Good Charlotte, bad diet for Joel Madden!

Nicole Richie‘s butter — er, better — half shed his shirt at a kids’ birthday party on April 2 in Palm Beach, Fla., and showed off a seriously fuller figure as he played on the inflatable water slide.

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“Nicole looked slim as ever, but Joel has clearly put on a lot of weight,” quipped a witness. “His tattoos were all distorted!”

Indeed, Star’s expert, weight-loss coach Alicia Hunter, author of The Accidental Diet, says the 5-foot-7 father of two needs to hit the gym.

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“Joel appears to be clocking in at 200 or 2010 pounds, which puts him in the obese range,” surmised Hunter. “For his height, his ideal weight is 140 to 155 pounds.”

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