Both Panasonic FZ1000 and Olympus E-M1 II have 20.0 MP resolution sensors but Panasonic FZ1000'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 Panasonic FZ1000.
Below you can see the FZ1000 and E-M1 II sensor size comparison.
As seen above, Olympus E-M1 II has a 1.9x Larger sensor area than Panasonic FZ1000. 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.