Both Fujifilm X20 and Pentax P80 have 12.0 MP resolution sensors but Fujifilm X20's sensor is 2/3-inch (8.8 x 6.6 mm ) and Pentax P80's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Fujifilm X20'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 X20 and P80 sensor size comparison.
As seen above, Fujifilm X20 has a 2.1x 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.