Both Panasonic ZS100 and Panasonic GX8 have 20.0 MP resolution sensors but Panasonic ZS100's sensor is 1" (13.2 x 8.8 mm ) and Panasonic GX8's sensor is Four Thirds (17.3 x 13 mm ).
Since Panasonic GX8'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.
Below you can see the ZS100 and GX8 sensor size comparison.
As seen above, Panasonic GX8 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.