[PapaBear3625]

Smaller ships are traditionally part of the Coast Guard.

The cutters, with crew of 2 officers, 20 crew, are much cheaper to deploy than destroyers,

[Carry_Okie]

The cutters, with crew of 2 officers, 20 crew, are much cheaper to deploy than destroyers,

PT Boats and Swift Boats were smaller yet, but they were part of the Navy. Size doesn't matter. It's the function that matters.

From Brave AI:

Barring icebreakers, the largest Coast Guard cutters are the 418-foot (127.4 m) Legend-class National Security Cutters (NSCs), which displace 4,500 tonnes and accommodate a crew of 120. These vessels, also known as Bertholf-class, serve as the largest combat ships in the fleet, designed for long-duration high-seas operations. Boats that big would usually be part of the Navy, but they do have a different function and are therefore not heavily armed or defended.