I have a feeling people are vastly overestimating how large the Leviathan ship is, they always note how it's large enough to swallow planets, but it's not.
Nessus is the celestial body that Calus intended to have the Leviathan swallow but Nessus isn't a planet, it isn't even close to being a planet as its closer in size to a large comet. And as we can see from the Nessus skybox the Leviathan is maybe 2 - 2 1/2 times the height of Nessus.
Compared to the Hive Dreadnaught which literally is visible in Saturn's rings making it close to the same length as HALF OF THE ENTIRE EARTH. The Leviathan simply cannot compare to a ship as large as the Dreadnaught, which Bungie very poorly represented it's actual scale in the cutscenes and in our time exploring it.
