Both Panasonic ZS100 and Panasonic ZS70 have 20.0 MP resolution sensors but Panasonic ZS100's sensor is 1" (13.2 x 8.8 mm ) and Panasonic ZS70's sensor is 1/2.3" (6.17 x 4.55 mm ).
Since Panasonic ZS100'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 ZS70.
Below you can see the ZS100 and ZS70 sensor size comparison.
As seen above, Panasonic ZS100 has a 4.1x Larger sensor area than Panasonic ZS70. 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.