Both Sony HX400V and Canon G3 X have 20.0 MP resolution sensors but Sony HX400V's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Canon G3 X's sensor is 1″ (13.2 x 8.8 mm ).
Since Canon G3 X'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 HX400V and G3 X sensor size comparison.
As seen above, Canon G3 X 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.