Nikon D80 has a
10.0MP APS-C (23.6 x 15.8 mm ) sized CCD 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 D80 and K100D Super sensor size comparison.
Nikon D80 and Pentax K100D S have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.