Both Ricoh WG-6 and Olympus E-M1 III have 20.0 MP resolution sensors but Ricoh WG-6's sensor is (1/2. ) and Olympus E-M1 III's sensor is Four Thirds (17.4 x 13 mm ).
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 WG-6.
One other difference between these two cameras that is worth mentioning is that Olympus E-M1 III's sensor doesn't have an 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 WG-6 and E-M1 III sensor size comparison.
As seen above, Olympus E-M1 III has a infx Larger sensor area than Ricoh WG-6. 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.