The Girls Aloud star told Zest that she "had the biggest bum out of about 50 girls in my class" and was not sure how to dress until Destiny's Child became famous.
Walsh said of Beyoncé: "We were exactly the same age and she looked amazing, so I started to dress like her and suddenly my shape made sense.
"I'd think, 'Oh, yes. Beyoncé's wearing a figure-hugging dress and drawing attention to her tiny waist', so I'd buy a new dress and a large belt, go out and pretend to be Beyoncé. She was a great alter ego for me."
Walsh last month said that she would like to perform a Beyoncé melody on musical comedy drama Glee.
The singer has previously complained that she finds it difficult to find trousers that fit because she has "a bigger bum and smaller waist".
Watch the Aggro Santos featuring Kimberley Walsh 'Like U Like' music video below: