Lifting an enclosed space is not free. Gravity is still in play so "huge rooms" means more floor space. Floor space is surface area and material costs by the square foot or meter both in terms of construction costs and in terms of weight.

On top of that, any time you're moving through a fluid your speed is determined by your thrust/drag ratio. When adding more space internally you have parasitic drag and induced drag working against you. Parasitic drag increases as you add surface area and when you grow your forward facing profile. Induced drag increases with the amount of lift you need to fly, more sq.ft, more material weight, more lift needed, more induced drag. Airships mostly fly with lifting gas but still generate a lot of body lift once they start moving.

So if you'd like to go anywhere once you get off the ground, you're going to need to balance cabin size against the size of the engines needed to produce enough thrust to travel at a reasonable speed along with the fuel weight required to travel any distance.