Both Canon M6 and Panasonic S1 have 24.0 MP resolution sensors but Canon M6's sensor is APS-C (22.3 x 14.9 mm ) and Panasonic S1's sensor is Full frame (35.6 x 23.8 mm ).
Since Panasonic S1'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 Canon M6.
One other difference between these two cameras that is worth mentioning is that Panasonic S1's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the M6 and S1 sensor size comparison.
As seen above, Panasonic S1 has a 2.5x Larger sensor area than Canon M6. 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
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Canon M6 and Panasonic S1 sensors have been tested by DxO and the results show that S1 has a better overall score of 95, 17 points higher compared to M6's score of 78.