Both Nikon D100 and Nikon D70 have 6.0 MP resolution sensors but Nikon D100's sensor is APS-C (23.7 x 15.5 mm ) and Nikon D70's sensor is APS-C (23.7 x 15.5 mm ).
Since Nikon D70'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 D100.
Below you can see the D100 and D70 sensor size comparison.
Nikon D100 and Nikon D70 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.