Sigma DP2 Quattro has a
20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS (Foveon X3) sensor and features TRUE III engine processor. On the other hand, Panasonic G85 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
One other difference between these two cameras that is worth mentioning is that Panasonic G85'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 DP2 Quattro and G85 sensor size comparison.
As seen above, Sigma DP2 Quattro has a 1.6x Larger sensor area than Panasonic G85. 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.