How does Carrie Underwood maintain her envied figure? It's all about time management.

"I don't really have time to eat junk food," the svelte star tells OK! magazine, adding that she does take time out to stay active. "I work out four to six days a week. Even if I don't do a lot, I just do cardio. It makes me feel better."

Underwood admits she goes "off the wagon" from time to time with her diet, but hops right back on to fit into awards show gowns, stage costumes and other unforgiving frocks. She also says that no matter what she's wearing, she stays fit for health reasons, not vanity.

"I don't think -- for me, at least -- there's a pressure to be thin. I think there's a pressure to be healthy," says the 26-year-old singer. "Last year, we did 148 live shows. We were out on the road for months at a time, so it was a pressure to be healthy for myself, because if I wasn't, then I'd get sick or have a lot of problems when I was on the road."

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While Underwood has been praised for her gorgeous gams lately, fellow country songbird Jessica Simpson hasn't been so lucky. After photos emerged of Simpson appearing more curvaceous, many speculated that she'd gained a lot of weight. Underwood doubts Simpson put on as many pounds as people think -- if any at all!

"I'm 5'3," so if I put on five pounds, it looks like a lot more than it actually is," Underwood insists. "I think she's [Jessica] shorter than I am. The first thing I hear people say when they meet me is, 'Oh my gosh, you're so short, you're so little,' because everybody looks so much bigger onstage and on TV. I think it's kind of unfair."

Underwood and Simpson are both set to appear on the 44th annual ACM Awards, live from Las Vegas on Sunday, April 5 at 8 PM ET.