Both Pentax K-S2 and Canon 6D have 20.0 MP resolution sensors but Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ) and Canon 6D's sensor is Full frame (36 x 24 mm ).
Since Canon 6D'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 6D sensor size comparison.
As seen above, Canon 6D has a 2.4x Larger sensor area than Pentax K-S2. 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.