Both Canon 400D and Olympus E-400 have 10.0 MP resolution sensors but Canon 400D's sensor is APS-C (22.2 x 14.8 mm ) and Olympus E-400's sensor is Four Thirds (17.3 x 13 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 Olympus E-400.
Below you can see the 400D and E-400 sensor size comparison.
As seen above, Canon 400D has a 1.5x Larger sensor area than Olympus E-400. 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.