Both Canon SX510 HS and Olympus E-P3 have 12.0 MP resolution sensors but Canon SX510 HS's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Olympus E-P3's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-P3'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 SX510 HS.
Below you can see the SX510 HS and E-P3 sensor size comparison.
As seen above, Olympus E-P3 has a 8.0x Larger sensor area than Canon SX510 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.