"She has always been thin," Knightley's mother, Sharman Macdonald, told Britain's Sunday Times. "She's her daddy's daughter, with his long body."
Like her father, the actress, 23, must "eat like a horse," to maintain her weight, Macdonald said. Knightley eats a diet high in protein and carbohydrates to make sure she doesn't lose any more weight.
Last year, Knightley successfully sued Britain's Daily Mail, saying that the tabloid implied she'd lied about having an eating disorder. In 2006, Knightley vociferously denied that she suffered from anorexia.
"I don't have it. I'm very sure I don't have it," she said at the Pirates of the Caribbean: Dead Man's Chest London premiere.
In the Times interview, Knightley's mother said she found the rumors upsetting, especially because she felt there was little she could do about them. "It's a playground situation that we're looking at with the press – it's a form of bullying – but what can we do?"