Both Nikon D750 and Sony A77 II have 24.0 MP resolution sensors but Nikon D750's sensor is Full frame (35.9 x 24 mm ) and Sony A77 II's sensor is APS-C (23.5 x 15.6 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 Sony A77 II.
Below you can see the D750 and A77 II sensor size comparison.
As seen above, Nikon D750 has a 2.4x Larger sensor area than Sony A77 II. 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.