There is too much contradictory information on the Leviathan. I haven't read any of the supplemental stories to the games, but I believe the Leviathan was supposed to be a Ravager styled ship, it was just never shown as such. Here are some reasons:
1) It was supposedly Admiral Karaths ship throughout the Mandalorian wars, in the Cutscenes it was shown to be a StarForge produced vessel, which wouldn't have been possible since Revan found the Starforge after the end of the Mandalorian Wars.
2) It was supposed to be one of the largest and most powerful vessels in the Sith fleet, yet appears identical to all other StarForge ships.
3) There are what appears to be the remains of another vessel of the same class as the Ravager in orbit around Malachor V, which shows the Ravager was not a unique vessel.
4) It is common practice for Admirals to captain the largest, most powerful ships in the fleet. So it is reasonable that Admirals captained Ravager class vessels opposed to the Republics other ships, which appear to be frigates.
5) It was stated by Carth that some of the ships in Malaks fleet were former Republic ships, but many were of an alien design ie StarForge vessels. You only ever see Sith controlling Star Forge ships - likely to keep cutscenes simple and unconfusing. But it raises the question; What is cannon? The in-game dialogue or a cutscenes image?
In short, I think they might very well be the same class of ship and therefore be equal in battle. At the very least it is far from certain that the Leviathan was a StarForge vessel, despite its appearance in the game.
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Those who understand binary and those who don't.
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