Canon 1D X has a 18.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual Digic 5+ processor. 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 1D X and 1Ds MIII sensor size comparison.
Canon 1D X 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 1D X has 17% 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.