Both Sony RX10 III and Sony HX400V have 20.0 MP resolution sensors but Sony RX10 III's sensor is 1 (13.2 x 8.8 mm ) and Sony HX400V's sensor is 1/2.3 (6.17 x 4.55 mm ).
Since Sony RX10 III'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 HX400V.
Below you can see the RX10 III and HX400V sensor size comparison.
As seen above, Sony RX10 III has a 4.1x Larger sensor area than Sony HX400V. 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.