Ship designations are more based in function, than displacement. Frigates were traditionally designed to operate independently from fleets, where destroyers are generally escorts for fleet vessels, like transports, cruisers and carriers.
Cruisers were traditionally designed with specific missions in mind, air defence cruisers(Ticonderoga), anti-ship cruisers (Slava), etc
But cruisers are going out of fashion, with navy’s opting for more capable Frigates and Destroyers over time, which is why the displacement has been growing.