Both Sony A99 and Sony A77 II have 24.0 MP resolution sensors but Sony A99's sensor is Full frame (35.8 x 23.8 mm ) and Sony A77 II's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A99'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 A77 II.
Below you can see the A99 and A77 II sensor size comparison.
As seen above, Sony A99 has a 2.3x Larger sensor area than Sony A77 II. 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 A99 and Sony A77 II 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, A99 has scored 89, 7 points higher than A77 II.