Both Panasonic FZ300 and Canon SX20 IS have 12.0 MP resolution sensors but Panasonic FZ300's sensor is (1/2. ) and Canon SX20 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon SX20 IS'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 SX20 IS sensor size comparison.
As seen above, Canon SX20 IS has a infx 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.