The logic behind both is essentially the same, as they're both just meant to be a striking visual. Indeed the former started life as a floating prison complex over the Imperial Capital planet Alderaan (second or third draft I think?) so the image came well before the function.
But yeah, the sheer size of that chamber just off of the hangar defies all logic. Best I can do is turn my head, squint, and pretend there's a deleted second part of the fight. One where they're riding a large cargo platform deep down under the palace to where the plasma mining complex that powers the city is located.
