Both Sony A6400 and Sony A7 II have 24.0 MP resolution sensors but Sony A6400's sensor is APS-C (23.5 x 15.6 mm ) and Sony A7 II's sensor is Full frame (35.8 x 23.9 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 A6400 and A7 II 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
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Sony A6400 and Sony A7 II sensors have been tested by DxO and the results show that A7 II has a better overall score of 90, 7 points higher compared to A6400's score of 83.