Both Panasonic ZS3 and Ricoh GR Digital III have 10.0 MP resolution sensors but Panasonic ZS3's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Ricoh GR Digital III's sensor is 1/1.7-inch (7.44 x 5.58 mm ).
Since Ricoh GR Digital III'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 Panasonic ZS3.
Below you can see the ZS3 and GR Digital III sensor size comparison.
As seen above, Ricoh GR Digital III has a 1.5x Larger sensor area than Panasonic ZS3. 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.