Both Nikon D70 and Nikon D100 have 6.0 MP resolution sensors but Nikon D70's sensor is APS-C (23.7 x 15.5 mm ) and Nikon D100's sensor is APS-C (23.7 x 15.5 mm ).
Since Nikon D100'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 Nikon D70.
Below you can see the D70 and D100 sensor size comparison.
Nikon D70 and Nikon D100 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.