Both Casio EX-10 and Ricoh GXR GR Lens A12 28mm have 12.0 MP resolution sensors but Casio EX-10's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Ricoh GXR GR Lens A12 28mm's sensor is APS-C (23.6 x 15.7 mm ).
Since Ricoh GXR GR Lens A12 28mm'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 Casio EX-10.
Below you can see the EX-10 and GXR GR Lens A12 28mm F2.5 sensor size comparison.
As seen above, Ricoh GXR GR Lens A12 28mm has a 8.9x Larger sensor area than Casio EX-10. 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.