ISD's take on the role of Destroyers in the Star Wars universe. They are heavily outgunned and out ranged by other imperial ships, so therefore take on a "destroyers" designation. Though they are massive and powerful ships on their own, in comparison to the Imperial navy they are standard sahips, that are outclasses by many others. You cant compare them to anything we have now in the world ,because the Star Wars universe has a different set of rules. We look at a ISD and go,"OOOOHHH!" but to people in the SW universe it's just another sight. Just like we have destroyers now, that we look at at go, "yeah it's kind of a small ship", but put that ship into combat 150 years ago, and everyones going to go'"OOOOOOOHHH!!!", but to us it's still just a dinky ship compared to a carrier orBB.