Well, one thing about classifying a Miranda as a Frigate, is you have to remember that by the time of DS9, the design is over 80 years old. It may have debuted as a Cruiser, but the ship was apparently designed to be easily refitted. Which is why it would still be in service maybe 100 years after it first launched. How many WWI era ships (even if they could be refitted with modern technology) would retain their previous classification? Would a WWI era Cruiser still be able to keep up with even a Cold War era Cruiser? Probably not, there's only so much refitting you can do to older ships. Federation designs got bigger, and even the Excelsior, a behemoth at the time of its launch, is itself dwarfed by the Ambassador and Galaxy Classes. Over time the Miranda, like all Starships, would eventually see a degradation in their classifications. From Cruiser, to Light Cruiser, to eventually Destroyer or Frigate, the versatility of the Miranda to still be a somewhat effective ship even a century after it's initial launching is a testament to the ship's design and foresight of the engineers. (And the cost consciousness of the Show's producers, who spent all that money on building and designing the Reliant, and didn't want it to be a one time showing)