Both Canon SX20 IS and Ricoh GXR GR Lens A12 28mm have 12.0 MP resolution sensors but Canon SX20 IS's sensor is 1/2.3-inch (6.17 x 4.55 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 Canon SX20 IS.
Below you can see the SX20 IS and GXR GR Lens A12 28mm F2.5 sensor size comparison.
As seen above, Ricoh GXR GR Lens A12 28mm has a 13.2x Larger sensor area than Canon SX20 IS. 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.