Both Sony RX10 IV and Sigma DP2 Quattro have 20.0 MP resolution sensors but Sony RX10 IV's sensor is 1″ (13.2 x 8.8 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 Sony RX10 IV.
Below you can see the RX10 IV and DP2 Quattro sensor size comparison.
As seen above, Sigma DP2 Quattro has a 3.2x Larger sensor area than Sony RX10 IV. 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.