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, Olympus E-M5 II has a
16.0MP Four Thirds (17.3 x 13 mm ) sized MOS sensor and features TruePic VII processor.
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 E-M5 Mark II sensor size comparison.
Olympus E-M1 III and Olympus E-M5 II have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.