Both Panasonic G9 and Sigma DP2 Quattro have 20.0 MP resolution sensors but Panasonic G9's sensor is Four Thirds (17.3 x 13 mm ) and Sigma DP2 Quattro's sensor is APS-C (23.5 x 15.7 mm ).
Since Sigma DP2 Quattro'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.
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 moiré occurring in certain scenes.
Below you can see the G9 and DP2 Quattro sensor size comparison.
As seen above, Sigma DP2 Quattro 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.