Both Leica SL2-S and Nikon D750 have 24.0 MP resolution sensors but Leica SL2-S's sensor is Full frame (36 x 24 mm ) and Nikon D750's sensor is Full frame (35.9 x 24 mm ).
Since Leica SL2-S'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 Nikon D750.
Another difference between these two cameras is that Leica SL2-S'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 moiré occurring in certain scenes.
Below you can see the SL2-S and D750 sensor size comparison.
Leica SL2-S and Nikon D750 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.