Both Casio EX-10 and Pentax W90 have 12.0 MP resolution sensors but Casio EX-10's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Pentax W90's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Casio EX-10'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 W90.
Below you can see the EX-10 and W90 sensor size comparison.
As seen above, Casio EX-10 has a 1.5x Larger sensor area than Pentax W90. 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.