Both Canon SD780 IS and Casio EX-10 have 12.0 MP resolution sensors but Canon SD780 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Casio EX-10's sensor is 1/1.7-inch (7.44 x 5.58 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 Canon SD780 IS.
Below you can see the SD780 IS and EX-10 sensor size comparison.
As seen above, Casio EX-10 has a 1.5x Larger sensor area than Canon SD780 IS. 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.