Both Olympus E-M1 III and Ricoh G900 have 20.0 MP resolution sensors but Olympus E-M1 III's sensor is Four Thirds (17.4 x 13 mm ) and Ricoh G900's sensor is (1/2. ).
Since Olympus E-M1 III's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Ricoh G900.
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 G900 sensor size comparison.
As seen above, Olympus E-M1 III has a infx Larger sensor area than Ricoh G900. 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.