Both Canon S120 and Panasonic ZS50 have 12.0 MP resolution sensors but Canon S120's sensor is 1/1.7" (7.44 x 5.58 mm ) and Panasonic ZS50's sensor is 1/2.3 (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 ZS50.
Below you can see the S120 and ZS50 sensor size comparison.
As seen above, Canon S120 has a 1.5x Larger sensor area than Panasonic ZS50. 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.
DxOMark Sensor Scores
Both Canon S120 and Panasonic ZS50 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, S120 has scored 56, 12 points higher than ZS50.