Both Canon A495 and Canon 400D have 10.0 MP resolution sensors but Canon A495's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Canon 400D's sensor is APS-C (22.2 x 14.8 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 Canon A495.
Below you can see the A495 and 400D sensor size comparison.
As seen above, Canon 400D has a 11.7x Larger sensor area than Canon A495. 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.