The swimsuit top featured a knotted front design while the bottoms were designed with a high-waisted fit. (source)
I thought it meant "a matching piece" but can't be sure because it appears to have some fashion-specific meaning.
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Here it means how the clothes were designed and made to fit on the body.
A slim fit jacket
Would be a suit that was designed for a slender person (for the same chest size, the slim fit jacket has a narrower waist.)
In your case the swimsuit is designed to fit high on the waist (and so probably expose more of the leg)