Both Canon S120 and Panasonic ZS15 have 12.0 MP resolution sensors but Canon S120's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Panasonic ZS15'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 Panasonic ZS15.
Below you can see the S120 and ZS15 sensor size comparison.
As seen above, Canon S120 has a 1.5x Larger sensor area than Panasonic ZS15. 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.