Both Canon T7i and Sony A99 have 24.0 MP resolution sensors but Canon T7i'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 T7i.
Below you can see the T7i and A99 sensor size comparison.
As seen above, Sony A99 has a 2.6x 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.