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.
One other difference between these two cameras that is worth mentioning is that Pentax K-S2's sensor doesn't have an 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 providing control over depth of field when used with same focal length and aperture.