The reality TV judge revealed the Oliver! producer is concerned by Prenger's curvaceous figure.
"Cameron thinks she is a bit too big and has more or less said so," he told The Mirror.
However, Webber defended Prenger, arguing that she has the vocal talents and "personality" to take on the role.
He commented: "Jodie could be anybody's idea of Nancy - I can see it absolutely. She has got a lovely voice and a super personality. She has experience as well."
The final of I'd Do Anything airs on Saturday on BBC1.