Conclusion: ships gets larger to fulfill more roles as older ships become obsolete, as militaries abhor vacuums.
That is one way of looking at it, another is that as higher tech makes ships more expensive, they become smaller to keep numbers up.
Classifications are just names that vary with time and navy.
Destroyer, ~300 t. Main armament: Torpedoes.
Destroyer, 1800 t. Main Armament: Torpedoes, Guns, AA, Depth Charges.
Destroyer, ~7500 t. Main Armament: Aegis anti-missile.
Destroyer, ~14000 t. Main Armament: Helicopters.
They are all classified as Destroyers by the same navy, yet they vary wildly in size, armament, intended use, and capability. Classification means next to nothing.
When we compare different navies classifications means even less:
Current Corvette, ~1800 t
Current Corvette, ~600 t