Some metaphors do more than substitute a lexical label: they create a rhetorical identification. A listener interprets the image rather than treating it as an external physical fact.
“He is an ocean of generosity” does not transfer seawater into a human being. It transfers breadth or abundance to the quality of giving. The human subject and reference to generosity establish the interpretation.
Without a defensible common quality, even a striking identification remains rhetorically weak. Exaggeration magnifies an intelligible relation; it cannot replace one.