I think I explained how FS classification system got how it was once before...
They called cruisers that because they were of similar size and function. Then someone made a ship meant to waltz in a blow your cruiser group to Kingdom Come, and called it a "cruiser destroyer". This was eventually shortened to "destroyer". (Which is a lot like how the modern destroyer ended up named what it was, actually. They used to be "torpedo-boat destroyers" or "battleship destroyers".)
Needing designations for their new overgrown cruisers and pocket destroyers, they turned to classic naval designations, and basing them from destroyer scale, they chose to call the new pocket destroyers "frigates", as frigates are the next ship down from classic destroyers. Corvettes are generally considered to be just below that, so that's what the overgrown cruisers were called.
See? It all makes sense.