Dropped 3 sizes! "I was determined," the 21-year-old singer tells PEOPLE about the motivation behind her successful bikini slim-down. After trying and failing with fad diets, the 5 ft., 10 in. former American Idol winner adopted a more realistic long-term approach, taking up Zumba classes and scaling down her portion sizes. "I've always felt cute, but now I feel sexier," she says.