Both Sony RX10 III and Olympus E-M1 II have 20.0 MP resolution sensors but Sony RX10 III's sensor is 1" (13.2 x 8.8 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 Sony RX10 III.
Below you can see the RX10 III and E-M1 II sensor size comparison.
As seen above, Olympus E-M1 II has a 1.9x Larger sensor area than Sony RX10 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.