07/06/2012 at 02:30 PM EDT
As Miley Cyrus
has been revealing more of her newly toned body in midriff tops and sexy LBDs
, critics have been getting more concerned about her shrinking waistline.
But Cyrus's Pilates instructor Mari Winsor, who has been working with the recently engaged
19-year-old star over the past four months, says there's no cause for concern.
"I think people need to chill out," says Winsor. "She's healthy and she feels good, that's the main thing. She's smart about her body. She wanted to sculpt her body, that's all."
To get her toned physique, Cyrus has also been following a gluten-free diet and going to Winsor's classes in Studio City, Calif., almost daily for one-hour sessions of circuit Pilates, which includes work on the reformer, mat exercises and stretching.
"What we focus on with Miley is posture. That was my main concern when she first came to me," says Winsor, who showcases the same exercises she does with Cyrus on her DVD Lower Body Pilates
. "I wanted to get her shoulders down and get her to relax her upper body. If you notice, her posture is a lot better."
"She works hard," adds Winsor. "When this girl comes in, she is on fire. She's focused."
The hard work has paid off: "She loves her abs, of course!" says Winsor. "She's young and she's got a beautiful body."