Both Sony A7R III and Sony A99 II have 42.0 MP resolution sensors but Sony A7R III's sensor is Full frame (35.9 x 24 mm ) and Sony A99 II's sensor is Full frame (35.9 x 24 mm ).
Since Sony A99 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 A7R III.
Another similarity between these two cameras is that both Sony A99 II and Sony A7R III sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the A7R III and A99 II sensor size comparison.
Sony A7R III and Sony A99 II have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Sony A7R III and Sony A99 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, A7R III has scored 100, 8 points higher than A99 II.