Olympus E-M1 III has a
20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic IX processor. On the other hand, Canon 1Ds MIII has a
21.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Another difference between these two cameras is that Olympus E-M1 III'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 E-M1 III and 1Ds MIII sensor size comparison.
As seen above, Canon 1Ds MIII has a 3.8x Larger sensor area than Olympus E-M1 III. 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.