Both Sony A9 and Nikon D750 have Full frame sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Sony A9'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 moire occurring in certain scenes.
Below you can see the A9 and D750 sensor size comparison.
Sony A9 and Nikon D750 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.