Both Canon 400D and Sony S2000 have 10.0 MP resolution sensors but Canon 400D's sensor is APS-C (22.2 x 14.8 mm ) and Sony S2000's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon 400D'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 S2000.
Below you can see the 400D and S2000 sensor size comparison.
As seen above, Canon 400D has a 11.7x Larger sensor area than Sony S2000. 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.