The object carries its own state. Its silhouette breathes and it takes a soft terracotta wash, with no bounding box anywhere.
The silhouette is clipped to the viewport, and the animation drives stroke opacity and fill opacity only.
Size-independent by construction. Case 4 reads exactly as well as case 1, which no box-based option manages.
Animating anything other than opacity on a large fill risks a repaint per frame. Opacity composites; geometry does not.