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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Canon 750D and Nikon D750 sensors have been tested by DxO and the results show that D750 has a better overall score of 93, 22 points higher compared to 750D's score of 71.