Both Pentax K-S1 and Pentax K-S2 have APS-C sized 20.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Pentax K-S2's sensor lacks anti-alias (Low-Pass) filter. 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-S1 and K-S2 sensor size comparison.
Pentax K-S1 and Pentax K-S2 sensors have exactly the same size so none of them has any advantage over other in this area.