Both Nikon D2X and Nikon D3S have 12.0 MP resolution sensors but Nikon D2X's sensor is APS-C (23.7 x 15.7 mm ) and Nikon D3S's sensor is Full frame (36 x 23.9 mm ).
Since Nikon D3S'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 D2X.
Below you can see the D2X and D3S sensor size comparison.
As seen above, Nikon D3S has a 2.3x Larger sensor area than Nikon D2X. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Nikon D2X and Nikon D3S sensors have been tested by DxO and the results show that D3S has a better overall score of 82, 23 points higher compared to D2X's score of 59.