Both Olympus E-620 and Pentax K-x have 12.0 MP resolution sensors but Olympus E-620's sensor is Four Thirds (17.3 x 13 mm ) and Pentax K-x's sensor is APS-C (23.6 x 15.8 mm ).
Since Pentax K-x'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 Olympus E-620.
Below you can see the E-620 and K-x sensor size comparison.
As seen above, Pentax K-x has a 1.7x Larger sensor area than Olympus E-620. 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.