Both Nikon S620 and Olympus E-PL2 have 12.0 MP resolution sensors but Nikon S620's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Olympus E-PL2's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-PL2'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 S620.
Below you can see the S620 and E-PL2 sensor size comparison.
As seen above, Olympus E-PL2 has a 8.1x Larger sensor area than Nikon S620. 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.