In order to transfer that bending moment developed at the bottom of the slab, Main reinforcement bar is to provide at the shorter span direction. The purpose of the main bar is to transfer the bending load developed at the bottom of the slab to the beams.
We place main reinforcement bars at the bottom of the slab.
Also, use slightly stronger (higher) dimension bars.
We place main bars at the shorter direction.
In one way slab, the slab is supported at two parallel sides where main reinforcement will be placed.
In two way slabs, the span will be supported at four ends. So there won’t be any difference in bar size. Because each side will have to transfer the same amount of stress evenly.