Both Samsung NX3000 and Panasonic G9 have 20.0 MP resolution sensors but Samsung NX3000's sensor is APS-C (23.5 x 15.7 mm ) and Panasonic G9's sensor is Four Thirds (17.3 x 13 mm ).
Since Samsung NX3000'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 G9.
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 NX3000 and G9 sensor size comparison.
As seen above, Samsung NX3000 has a 1.6x Larger sensor area than Panasonic G9. 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.