Both Canon 77D and Sony A99 have 24.0 MP resolution sensors but Canon 77D's sensor is APS-C (22.3 x 14.9 mm ) and Sony A99's sensor is Full frame (35.8 x 23.8 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 Canon 77D.
Below you can see the 77D and A99 sensor size comparison.
As seen above, Sony A99 has a 2.6x Larger sensor area than Canon 77D. 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. Canon 77D and Sony A99 sensors have been tested by DxO and the results show that A99 has a better overall score of 89, 11 points higher compared to 77D's score of 78.