Open space in a spacecraft needs to be filled with air and climate controlled, so it's not like it's totally free to operate. On the other hand, it's a hell of a lot lighter than having the same volume full of stuff.
Thus, the real question is why bother making a ship bigger than it needs to be - and full of empty space - rather than smaller and packed in tight? There's all kinds of headcanon we can use here, such as the warp field working better with different topologies, or just providing plenty of space within the hull for upgrades if they are needed.
Thus, the real question is why bother making a ship bigger than it needs to be - and full of empty space - rather than smaller and packed in tight? There's all kinds of headcanon we can use here, such as the warp field working better with different topologies, or just providing plenty of space within the hull for upgrades if they are needed.