She told People.com: "I think people need to chill out. 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."
Mari added Miley's toned physique is the result of a gluten-free diet and daily Pilates classes.
She added: "What we focus on with Miley is posture. That was my main concern when she first came to me. 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. When this girl comes in, she is on fire. She's focused."
Miley got engaged to actor Liam Hemsworth on May 31 and is already planning her nuptials, although it is thought they will not be wed until next year.
A source told HollywoodLife.com: "Nothing has been definite yet. She's just pretty much brainstorming everything about her wedding. I know she wants to have a summer wedding, something on the water maybe."
