What gets removed
Editor metadata (Inkscape and Sketch namespaces, Illustrator comments), empty groups, default attribute values, hidden elements and redundant path commands. Coordinates are rounded to the precision you choose and paths are rewritten in their shortest form.
Precision is the big lever
Design tools often write coordinates like 12.3456789. Rounding to 1–2 decimals rarely changes anything visible on icons drawn at 24–512 px, but it can halve the file. Drop precision too far and curves start to wobble; the explainer below lets you see exactly where that happens.