Both Olympus E-PM1 and Sony S980 have 12.0 MP resolution sensors but Olympus E-PM1's sensor is Four Thirds (17.3 x 13 mm ) and Sony S980's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Olympus E-PM1'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 Sony S980.
Below you can see the E-PM1 and S980 sensor size comparison.
As seen above, Olympus E-PM1 has a 8.0x Larger sensor area than Sony S980. 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.