Both Canon S120 and Casio EX-G1 have 12.0 MP resolution sensors but Canon S120's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Casio EX-G1's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon S120'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-G1.
Below you can see the S120 and EX-G1 sensor size comparison.
As seen above, Canon S120 has a 1.5x Larger sensor area than Casio EX-G1. 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.