Coffer or waffle slabs weigh much less than conventional slabs and can span the same distance if properly reinforced.
The reinforced steel does most of the work and cement is cast in sutu around the reinforced steel system.
The ribs of the coffer slab act as beams spanning in two directions to help carry the load of the concrete slab.
The cavities between the ribs reduce the weight of the structure and require a lot less cement.