Both Nikon D800 and Pentax K-1 II have 36.0 MP resolution sensors but Nikon D800'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 D800.
One other difference between these two cameras that is worth mentioning is that Pentax K-1 II's sensor doesn't have an 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 D800 and K-1 Mark II sensor size comparison.
Nikon D800 and Pentax K-1 II have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.