Below you can see the front view size comparison of Pentax K-3 II and Nikon D850. Pentax K-3 II is clearly the smaller of the two cameras. Its body is 15mm narrower, 24mm shorter and 2mm thinner than Nikon D850.
Pentax K-3 II vs Nikon D850 - Front View
Pentax K-3 II 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