Both Canon T7i and Nikon D3300 have 24.0 MP resolution sensors but Canon T7i's sensor is APS-C (22.3 x 14.9 mm ) and Nikon D3300's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D3300'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.
One other difference between these two cameras that is worth mentioning is that Nikon D3300's sensor doesn't have an 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 moiré occurring in certain scenes.
Below you can see the T7i and D3300 sensor size comparison.
As seen above, Nikon D3300 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.