When a character has a sheath that is relative to themselves a lot larger than what is anatomically possible. But is still, to some extent, plausible looking. Do note that a sheath, being effectively a sleeve for a character's own cock and thus the diameter will often match that of the cock, is best measured primarily by its length. But still with some respect to the diameter if it's significantly thicker than what would be expected for its length. But not if the sheath only has a large diameter, while completely skipping out on any length at all. That's a common tagging mistake.
And generally if the sheath's length either exceeds about 1/2 of a feral or 2/5 of an anthro character's underside/belly & chest area. Or is close to that and is significantly thicker than expected. Then the tag hyper sheath is likely applicable. Related huge: