Given a number of factors to take into consideration it would make sense if it was a giant spacecraft. One, hewing the rock that was already existing in space and shaping it into the form of a spacecraft means materials arn't used to create the Hull, it is already there. Plus, having it in a single solid piece would mean that he hull has more integrity. All you would need to do after hollowing it out would be to outfit the inside with the electronics needed and attach the prepulsion system onto the back. This would also allow the ship to be much much larger than if it had to be constructed with 100% fabricated materials.