Both Pentax K-S2 and Olympus PEN-F have 20.0 MP resolution sensors but Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ) and Olympus PEN-F'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 Olympus PEN-F.
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-S2 and PEN-F sensor size comparison.
As seen above, Pentax K-S2 has a 1.6x Larger sensor area than Olympus PEN-F. 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.