The reality star, who is rumoured to be entering the house with mum Sue, has taken on a "gruelling" routine to shape up in time for the show's launch on Channel 5.
A source told the Daily Star Sunday: "She has always had battles when it comes to her weight but going into Big Brother has given her a goal and kept her motivated. Not only is she really slim at the moment, she's also really toned, with even some light muscles."
Meanwhile, Amy Childs, who quit The Only Way Is Essex for Celebrity Big Brother, is currently on holiday in Portugal.
"Amy has such a sexy body and no one could take their eyes off her when she stripped down to her bikini," one onlooker said. "When male housemates see her taking a dip in the Big Brother pool, they are going to want to get in with her. She's bound to be a hit with male viewers, looking like that."
A show source added: "With Amy and Kerry looking so sexy and with a few hunks going into the house, there's bound to be some flirting and probably more. Amy is newly single and on the hunt for a man, so there's a good chance we'll see romance."
It was previously reported that Katona was dieting over concerns she would look bigger next to Childs and Pamela Anderson - also said to be taking part.
Celebrity Big Brother will return on Thursday, August 18 at 9pm on Channel 5.
Watch the Channel 5 Big Brother trailer below: