Canon 1Ds MIII has a 21.0MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Canon 1Ds MII has a 17.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Below you can see the 1Ds MIII and 1Ds MII sensor size comparison.
Canon 1Ds MIII and Canon 1Ds MII 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 (52.02µm2 vs 41.08µm2) compared to Canon 1Ds MIII, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Canon 1Ds MIII and Canon 1Ds MII sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, 1Ds MIII has scored 80, 6 points higher than 1Ds MII.