Canon 400D has a 10.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor . On the other hand, Pentax K100D S has a 6.0MP APS-C (23.5 x 15.7 mm ) sized CCD sensor .
Below you can see the 400D and K100D Super sensor size comparison.
As seen above, Pentax K100D S has a 1.1x Larger sensor area than Canon 400D. 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.