Both Canon SL2 and Sony A7 have 24.0 MP resolution sensors but Canon SL2's sensor is APS-C (22.3 x 14.9 mm ) and Sony A7's sensor is Full frame (35.8 x 23.9 mm ).
Since Sony A7'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 SL2.
Below you can see the SL2 and A7 sensor size comparison.
As seen above, Sony A7 has a 2.6x Larger sensor area than Canon SL2. 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.