Both Pentax K-S2 and Pentax K-S1 have APS-C sized 20.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Pentax K-S1 and Pentax K-S2 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the K-S2 and K-S1 sensor size comparison.
Pentax K-S2 and Pentax K-S1 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.