Both Olympus E-PM1 and Sony A7S II have 12.0 MP resolution sensors but Olympus E-PM1's sensor is Four Thirds (17.3 x 13 mm ) and Sony A7S II's sensor is Full frame (35.6 x 23.8 mm ).
Since Sony A7S 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 Olympus E-PM1.
Below you can see the E-PM1 and A7S II sensor size comparison.
As seen above, Sony A7S II has a 3.8x Larger sensor area than Olympus E-PM1. 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.