Both Ricoh GXR GR Lens A12 28mm and Canon SX40 HS have 12.0 MP resolution sensors but Ricoh GXR GR Lens A12 28mm's sensor is APS-C (23.6 x 15.7 mm ) and Canon SX40 HS's sensor is 1/2.3-inch (6.17 x 4.55 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 SX40 HS.
Below you can see the GXR GR Lens A12 28mm F2.5 and SX40 HS sensor size comparison.
As seen above, Ricoh GXR GR Lens A12 28mm has a 13.2x Larger sensor area than Canon SX40 HS. 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.