I say by the mission. It's just as Navies enlarge the Armament and size, so some of the mission specs.
Just take example of what the European devinition of Next Gen Corvetes:
Spain seen joining Greece, France, Italy on European Patrol Corvette program
Not long ago 2000-2500 ton can still be consider Frigates or Light Frigates. 500 ton - 2000 ton consider to be Corvete and below 500 ton are Patrol Boats.
Now 3000 ton can be consider Corvetes. Corvetes dimension also increase from 70m-95m not long time ago toward 100m-115m even now 120m. That the range that usualy belong to Frigates.
Some duties/missions that used to be done by Frigates being done now by new type of corvettes, just as like some Destroyers mision being done by new type Frigate (some called them Heavy Frigates).
In short, everything enlarge. Thus Cruisers perhaps now in the range of 20,000 ton more like USSR originated Kirov class Cruisers, which used to ve criteria of 'Battle Cruisers'.