Both Nikon D5500 and Canon T7i have 24.0 MP resolution sensors but Nikon D5500's sensor is APS-C (23.5 x 15.6 mm ) and Canon T7i's sensor is APS-C (22.3 x 14.9 mm ).
Since Nikon D5500'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.
Another difference between these two cameras is that Nikon D5500's sensor lacks anti-alias (Low-Pass) filter. 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 D5500 and T7i sensor size comparison.
As seen above, Nikon D5500 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.