Both Pentax Q-S1 and Pentax Q10 have 12.0 MP resolution sensors but Pentax Q-S1's sensor is 1/1.7" (7.44 x 5.58 mm ) and Pentax Q10's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Pentax Q-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 Pentax Q10.
Below you can see the Q-S1 and Q10 sensor size comparison.
As seen above, Pentax Q-S1 has a 1.5x Larger sensor area than Pentax Q10. 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.