Why a small ship might carry a full captain or commodore

Why would a full captain be in charge of a small ship?

The officer may command and coordinate a whole group, as in complex postwar minesweeping operations.

A senior officer aboard a small ship may be commanding a larger operation. The responsibilities can include coordinating several vessels and directing their combined work. Post-World War II minesweeping is given as an example where complexity and the number of ships justified captains or commodores. A billet associated with a commander's small ship could therefore require a more senior officer for the operation it supported.