Both Nikon D2X and Casio EX-Z90 have 12.0 MP resolution sensors but Nikon D2X's sensor is APS-C (23.7 x 15.7 mm ) and Casio EX-Z90's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Nikon D2X'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 Casio EX-Z90.
Below you can see the D2X and EX-Z90 sensor size comparison.
As seen above, Nikon D2X has a 13.3x Larger sensor area than Casio EX-Z90. 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.