Leica SL2-S has a
24.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro III processor. On the other hand, Canon 1D X III has a
20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic X processor.
Another difference between these two cameras is that Leica SL2-S's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the SL2-S and 1D X MIII sensor size comparison.
Leica SL2-S and Canon 1D X III 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. On the other hand, since Canon 1D X III has 20% larger pixel area (43.29µm2 vs 36.00µm2) compared to Leica SL2-S, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.