Two tops can be labelled "medium" with the same body measurement target, yet one is tight and one is loose — the difference is how much ease the designer built in. A tailored shirt has little ease; an oversized one has a lot.
Negative ease only works with stretch fabrics: a bodycon dress is cut smaller than your body and depends on elastane to fit. Knowing a garment's intended ease tells you far more than the size label does.