Both Leica C-Lux and Panasonic G9 have 20.0 MP resolution sensors but Leica C-Lux's sensor is ( ) and Panasonic G9's sensor is Four Thirds (17.3 x 13 mm ).
Since Panasonic G9'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 Leica C-Lux.
One other difference between these two cameras that is worth mentioning is that Panasonic G9'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 moiré occurring in certain scenes.
Below you can see the C-Lux and G9 sensor size comparison.
As seen above, Panasonic G9 has a infx Larger sensor area than Leica C-Lux. 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.