Both Canon S120 and Nikon P300 have 12.0 MP resolution sensors but Canon S120's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Nikon P300's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon S120'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 Nikon P300.
Below you can see the S120 and P300 sensor size comparison.
As seen above, Canon S120 has a 1.5x Larger sensor area than Nikon P300. 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.