Both Casio EX-10 and Sony S2100 have 12.0 MP resolution sensors but Casio EX-10's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Sony S2100'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 Sony S2100.
Below you can see the EX-10 and S2100 sensor size comparison.
As seen above, Casio EX-10 has a 1.5x Larger sensor area than Sony S2100. 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.