Come to think of it, there is a certain logic to a ship design with that much empty space. If the ship is hulled by meteoroids, or by enemy fire, then the more empty space you have, the lower the probability of a populated section being hit. In space, with no air resistance to worry about, there's no cost to having the ship's volume be larger than it has to.
Except for the increased mass penalty, which would be an issue if the impulse engines function as Newtonian rockets
. A minimized exterior surface area would also provide statistical protection from natural hazards as well as presenting a smaller target to the enemy during combat.