Will the volume of concrete in the slab reduce after the final setting time?
Yes, but not by much to be appreciable.
Generally, concrete is known to shrink upon drying. This shrinkage causes lateral tensile forces in the concrete which results in cracks in the concrete. It is estimated that a concrete slab can shrink as much as 1/2 inch per 100 feet. Volumetrically the drying shrinkage is estimated at about 0.06 percent
The shrinkage however can be reduced by 50 percent by reducing the water content in the concrete to a water-cement ratio of 0.35.