Both Canon SX500 IS and Olympus E-PM2 have 16.0 MP resolution sensors but Canon SX500 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Olympus E-PM2's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-PM2'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 SX500 IS.
Below you can see the SX500 IS and E-PM2 sensor size comparison.
As seen above, Olympus E-PM2 has a 8.0x Larger sensor area than Canon SX500 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.