Actyually, I can. both of the above, while substantially different in scale, power, and technology, fulfil the same purpose: a multi-purpose massive capital ship meant to serve as a highly mobile base of operations, capable (though not necessarily used for) of fielding large ammounts of forces for a variety of missions including border guarding, ship-to-ship combat, and planetary invasion, and do all of those well. It is quite obvious that the Destroyer category in space is not the same as the Destroyer category at sea; starfighters are the small, fast, heavily armed vessels of deep space, so having a small capital ship class purely for adding offensive guns would be a waste, and dangerous; corvettes (which closest resemble water-bound destroyers, of all space vessels) are not good for providing main offensive firepower, even though that's all the rebellion had. Instead, they were meant as flexible escort to much larger capital ships, as well as well armed transports (a la the "blockade runner"). Destroyers in deep space, on the other hand, seem to be a combination of aircraft carriers, troopships, and battleships (which are an extinct category in the water). Even though the Victory-class star destroyer is but 900m long compared to the 17,800m of the Eclipse-class star destroyer, both perform the same deep-space function of covering all the bases expected for various capital ships with a single massive (from somewhat massive to massive beyond belief) ship.
Wake up, George Lucas... The Matrix has you...