Below you can see the front view size comparison of Nikon D750 and Nikon D850. Nikon D750 is clearly the smaller of the two cameras. Its body is 5mm narrower, 11mm shorter and 1mm thinner than Nikon D850.
Nikon D750 vs Nikon D850 - Front View
Nikon D750 vs Nikon D850 - Top View
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.
Camera Size Comparison
Select Two Cameras for Size Comparison