Canon 1Ds MII has a 17.0MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Canon 1Ds MIII has a 21.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Below you can see the 1Ds MII and 1Ds MIII sensor size comparison.
Canon 1Ds MII and Canon 1Ds MIII have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon 1Ds MII has 26% larger pixel area compared to Canon 1Ds MIII, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
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. Canon 1Ds MII and Canon 1Ds MIII sensors have been tested by DxO and the results show that 1Ds MIII has a better overall score of 80, 6 points higher compared to 1Ds MII's score of 74.