Both Ricoh WG-6 and Pentax K-S1 have 20.0 MP resolution sensors but Ricoh WG-6's sensor is 1/2.3 (6.17 x 4.55 mm ) and Pentax K-S1's sensor is APS-C (23.5 x 15.6 mm ).
Since Pentax K-S1'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 Ricoh WG-6.
One other difference between these two cameras that is worth mentioning is that Pentax K-S1'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 moire occurring in certain scenes.
Below you can see the WG-6 and K-S1 sensor size comparison.
As seen above, Pentax K-S1 has a 13.1x Larger sensor area than Ricoh WG-6. 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.