Both Ricoh WG-6 and Olympus E-M1 II have 20.0 MP resolution sensors but Ricoh WG-6's sensor is 1/2.3" (6.17 x 4.55 mm ) and Olympus E-M1 II's sensor is Four Thirds (17.4 x 13 mm ).
Since Olympus E-M1 II'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 II'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 moire occurring in certain scenes.
Below you can see the WG-6 and E-M1 II sensor size comparison.
As seen above, Olympus E-M1 II has a 8.1x 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.