Both Pentax K-S2 and Samsung NX20 have 20.0 MP resolution sensors but Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ) and Samsung NX20's sensor is APS-C (23.5 x 15.7 mm ).
Since Samsung NX20's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Pentax K-S2.
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 moiré occurring in certain scenes.
Below you can see the K-S2 and NX20 sensor size comparison.
Pentax K-S2 and Samsung NX20 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.