The 29-year-old feared that she would lose out in the show's final after Andrew Lloyd Webber went public with fears held by the musical's producer Cameron Mackintosh.
Prenger had dropped from a size 22 to a size 14 before plucking up the courage to try out for West End roles.
Writing in her new book It's A Fine Life: My Story, Prenger explained: "I desperately hoped the winner would be me. Yet my confidence had been knocked. The day before the final, Andrew Lloyd Webber had revealed to a newspaper that Cameron Mackintosh thought I was a bit 'too big'.
"When I read that, I was mortified. For years, I had battled with my weight. I was now a size 14, the average size for a woman, yet I was still being criticised. It really upset me and I started to think I might be out of the running."
Prenger went on to win I'd Do Anything, triumphing over Jessie Buckley in the show's final. She made her debut in Oliver! in January.