What if really the reason it looks so massive in concept art and in game is because it's next to something that appears large but really isn't? Assuming Nessus in Destiny is 60 KM in diameter which I believe is accurate, the leviathan is really quite small. The Drednaught using this logic would actually be bigger than the Leviathan, or around the same considering it looks that large next to Saturn which is 116,438 KM.
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It’s not right beside it, bit away from the planet itself so could account for size
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True, but even with that in mind it's still quite massive. Definitely bigger than Nessus, and I think it's possible that it could be bigger than the Leviathan.