Both Nikon P300 and Ricoh GXR Mount A12 have 12.0 MP resolution sensors but Nikon P300's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Ricoh GXR Mount A12's sensor is APS-C (23.6 x 15.7 mm ).
Since Ricoh GXR Mount A12'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 Nikon P300.
Below you can see the P300 and GXR Mount A12 sensor size comparison.
As seen above, Ricoh GXR Mount A12 has a 13.2x Larger sensor area than Nikon P300. 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.