Both Pentax WG-3 and Olympus E-M1 have 16.0 MP resolution sensors but Pentax WG-3's sensor is 1/2.3" (6.17 x 4.55 mm ) and Olympus E-M1's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-M1'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 Pentax WG-3.
Below you can see the WG-3 and E-M1 sensor size comparison.
As seen above, Olympus E-M1 has a 8.0x Larger sensor area than Pentax WG-3. 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.