Both Olympus E-PM1 and Pentax Q7 have 12.0 MP resolution sensors but Olympus E-PM1's sensor is Four Thirds (17.3 x 13 mm ) and Pentax Q7's sensor is 1/1.7-inch (7.44 x 5.58 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 Pentax Q7.
Below you can see the E-PM1 and Q7 sensor size comparison.
As seen above, Olympus E-PM1 has a 5.4x Larger sensor area than Pentax Q7. 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.