Both Samsung NX200 and Pentax K-S2 have 20.0 MP resolution sensors but Samsung NX200's sensor is APS-C (23.5 x 15.7 mm ) and Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ).
Since Samsung NX200'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.
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 moiré occurring in certain scenes.
Below you can see the NX200 and K-S2 sensor size comparison.
Samsung NX200 and Pentax K-S2 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.