When Kimberley Locke belted out "Over the Rainbow" during the second season of American Idol, it was crystal clear to the judges and viewers alike that her voice was big. But at 5'4" and 165 lbs., the second runner-up couldn't argue when Simon Cowell told her on air that her body shouldn't be oversize too. "He didn't really hurt my feelings," says the Tennessee native, 26, a former high school athlete who'd developed a fondness for pizza. "I knew I had to do something."

What the onetime law student needed was discipline, and she found it in L.A.-based trainer Lee Brandon. Since November, when Brandon began pushing Locke through a five-days-a-week gym regimen (half-hour cardio and an hour of weight training), the singer has lost at least 20 lbs. and added new layers of muscle that emphasize her curves. Now the novice pop star will be able to command the stage like a diva when she promotes her first album, due this spring. (Her first single, "8th World Wonder," is receiving radio airplay nationwide.) "We cut the waist in so it looks tiny," says Brandon, who has trained NFL players. "We've taken nearly an inch off each of her thighs and nearly 10 inches off her torso." In particular, says Locke, "I like that cut look on my arms."

That's not all that's cut; junk-food fat is gone from her diet except on her one cheat day a week. "When I go out, instead of ranch dressing I'll have vinaigrette," she says, "and ask for a tablespoon of ranch on the side." At home in L.A., she sticks to grilled chicken—even when her roomie Clay Aiken "eats Hot Pockets pizza for every meal," she jokes. "I just ignore that freezer full of junk."

Locke's goal is to be able to fit nicely in a size 8 (she's down to a 12 from a 14), but she's realistic. "I'll never be a size 0. It's not in my genes. I just don't want to jiggle anymore. Well, except for the places I want to jiggle."