Both Casio Pro EX-F1 and Fujifilm S3 Pro have 6.0 MP resolution sensors but Casio Pro EX-F1's sensor is 1/1.8" (7.144 x 5.358 mm ) and Fujifilm S3 Pro's sensor is APS-C (23 x 15.5 mm ).
Since Fujifilm S3 Pro'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 Pro EX-F1.
Below you can see the Pro EX-F1 and S3 Pro sensor size comparison.
As seen above, Fujifilm S3 Pro has a 9.3x Larger sensor area than Casio Pro EX-F1. 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.