Both Canon 750D and Nikon D750 have 24.0 MP resolution sensors but Canon 750D's sensor is APS-C (22.3 x 14.9 mm ) and Nikon D750's sensor is Full frame (35.9 x 24 mm ).
Since Nikon D750'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 750D.
Below you can see the 750D and D750 sensor size comparison.
As seen above, Nikon D750 has a 2.6x Larger sensor area than Canon 750D. 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.