Both Canon T7i and Sony A77 have 24.0 MP resolution sensors but Canon T7i's sensor is APS-C (22.3 x 14.9 mm ) and Sony A77's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A77'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 T7i.
Below you can see the T7i and A77 sensor size comparison.
As seen above, Sony A77 has a 1.1x Larger sensor area than Canon T7i. 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.