Both Sony A7 II and Sony A6400 have 24.0 MP resolution sensors but Sony A7 II's sensor is Full frame (35.8 x 23.9 mm ) and Sony A6400's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A7 II'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 A6400.
Below you can see the A7 II and A6400 sensor size comparison.
As seen above, Sony A7 II has a 2.3x Larger sensor area than Sony A6400. 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.
DxOMark Sensor Scores
Both Sony A7 II and Sony A6400 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, A7 II has scored 90, 7 points higher than A6400.