Both Pentax K-S2 and Panasonic G9 have 20.0 MP resolution sensors but Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ) and Panasonic G9's sensor is Four Thirds (17.3 x 13 mm ).
Since Pentax K-S2'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 Panasonic G9.
Another similarity between these two cameras is that both Panasonic G9 and Pentax K-S2 sensors lack anti-alias (Low-Pass) filters. 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-S2 and G9 sensor size comparison.
As seen above, Pentax K-S2 has a 1.6x Larger sensor area than Panasonic G9. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.