The difficulty in ship classification is that at the turn of the last century, it tended to be based around gross tonnage and/or no. guns. This shifted somewhat in later years to be based on speed, or thickness of armour, or weight of broadside.
In more modern terms, it is based on role. 'Destroyer' in a modern navy tends to refer to AAW vessels, rather than anything else. If you look at the RN for instance, the Type 22 Frigate is actually a vessel of equal or greater displacement than their contemporary Type 42 destroyers. They're also longer, have a broader beam and deeper draught. So why are Type 22's 'frigates' and not 'destroyers'? Why are Type 45 'destroyers' and not 'frigates'? Partly it's political, but as I pointed out, it very much hones in on their role in a fleet. AAW centric vessels are considered destroyers, ASW vessels frigates. Earth Alliance may take a similar tack.