Why were large red mullets so highly prized by the Roman epicures?

Because they were more difficult to be got; not that the larger fish were more delicate. Juvenal says,

The lavish slave Six thousand pieces for a mullet gave, A sesterce for each pound.

- But Apicius gave 8000 nummi, or 641. 11s. 8d. for a fish of as small a size as a mullet.