Both Olympus 1s and Panasonic FZ300 have 12.0 MP resolution sensors but Olympus 1s's sensor is 1/1.7" (7.44 x 5.58 mm ) and Panasonic FZ300's sensor is 1/2.3" (6.17 x 4.55 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 1s and FZ300 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.