Both Canon 400D and Pentax E70 have 10.0 MP resolution sensors but Canon 400D's sensor is APS-C (22.2 x 14.8 mm ) and Pentax E70's sensor is 1/2.3-inch (6.08 x 4.56 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 Pentax E70.
Below you can see the 400D and E70 sensor size comparison.
As seen above, Canon 400D has a 11.9x Larger sensor area than Pentax E70. 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.