Both Pentax P80 and Olympus E-PL1 have 12.0 MP resolution sensors but Pentax P80's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Olympus E-PL1's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-PL1'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 P80.
Below you can see the P80 and E-PL1 sensor size comparison.
As seen above, Olympus E-PL1 has a 8.0x Larger sensor area than Pentax P80. 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.