Both Panasonic FZ300 and Olympus 1s have 12.0 MP resolution sensors but Panasonic FZ300's sensor is 1/2.3" (6.17 x 4.55 mm ) and Olympus 1s's sensor is 1/1.7" (7.44 x 5.58 mm ).
Since Olympus 1s'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 FZ300.
Below you can see the FZ300 and 1s sensor size comparison.
As seen above, Olympus 1s has a 1.5x Larger sensor area than Panasonic FZ300. 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.