Pentax K-50 has a 16.0MP APS-C (23.7 x 15.7 mm ) sized CMOS sensor and features PRIME M processor. On the other hand, Pentax K-S2 has a 20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor.
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-50 and K-S2 sensor size comparison.
Pentax K-50 and Pentax K-S2 have almost the same sensor size so none of them has any significant advantage over other in this area.