Tailoring is about shaping, not just size: darts, seams, and a defined waist suppression give the garment a contour that follows the body. A tailored blazer nips in slightly at the waist; tailored trousers have a clean line through the hip and thigh without clinging.
Because a tailored fit is built around proportion, it's the cut most worth trying on — small differences in shoulder width or waist shaping change how sharp it looks on your frame.