Both Nikon D810 and Pentax K-1 II have 36.0 MP resolution sensors but Nikon D810's sensor is Full frame (35.9 x 24 mm ) and Pentax K-1 II's sensor is Full frame (35.9 x 24 mm ).
Since Pentax K-1 II'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 D810.
Another similarity between these two cameras is that both Pentax K-1 II and Nikon D810 sensors lack anti-alias (Low-Pass) filters. 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 D810 and K-1 Mark II sensor size comparison.
Nikon D810 and Pentax K-1 II have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.