ISDs are certainly tougher than the Sith ships of a similar design to the Leviathan. There was a fleet of ships that you can clearly see in the sequence where you're in the turret taking on the Sith fighters if you turn towards Taris you'll see a number of ships above Taris with explosions occuring on the surface of the planet. Also if you look at the weaponry shown on starwars.com for the Leviathan, its main weapons are composed of Quad laser cannons, not turbolasers like the ISD, so obviously a Star Destroyer could easily face up to a Leviathan-class ship. Really there is no doubt that an Imperial Star Destroyer could face down a Sith ship with relative ease, but for its day the Leviathan was a formidable warship.