Actually, the Leviathn is a Ratakan made ship. It's significantly smaller then a Star Destroyer. Also, in order to glass a planet, an Imperial fleet needed perhaps less then ten capital ships (no more than fifteen tops... I need to reread the Hutt's Gambit) to glass Nar Shadaa, killing all life. The Leviathan and its similar capital ships needed several hours to properly position themselves, and even then the bombardment came from a swarm of ships. As shown in the FMV, the turbolasers of the Leviathan, while powerful, did relatively light damage to the homes of Taris compared to the might of some of the larger capital ships in the times of the Empire.