Both Panasonic ZS100 and Panasonic G9 have 20.0 MP resolution sensors but Panasonic ZS100's sensor is 1 (13.2 x 8.8 mm ) 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 Panasonic ZS100.
Another difference between these two cameras is that Panasonic G9's sensor lacks 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 ZS100 and G9 sensor size comparison.
As seen above, Panasonic G9 has a 1.9x Larger sensor area than Panasonic ZS100. 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.