Pop beauty Jessica Simpson is paranoid about the size of her bottom and gets enraged when it is mocked by her Dukes Of Hazzard co-star Johnny Knoxville. The singer, who beat off Britney Spears for the coveted role of Daisy Duke in the big-screen remake, has reportedly had enough of other actors' jokey antics. An insider says, "They have been saying her butt looks big compared to the original Daisy. They know she's sensitive about it." Simpson allegedly snapped when 'Jackass' star Knoxville and American Pie actor Sean William Scott jumped out of a car to surprise her - naked from the waist down. The source adds, "Jessica told them to grow up. She was freaked out."