Both Ricoh G900 and Pentax K-S1 have 20.0 MP resolution sensors but Ricoh G900'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 G900.
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 G900 and K-S1 sensor size comparison.
As seen above, Pentax K-S1 has a 13.1x Larger sensor area than Ricoh G900. 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.