Nikon D780 has a
25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Pentax K-3 III Monochrome has a
26.0MP APS-C (23 x 15.5 mm ) sized CMOS sensor and features PRIME V processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-3 III Monochrome'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 D780 and K-3 III Monochrome sensor size comparison.
As seen above, Nikon D780 has a 2.4x Larger sensor area than Pentax K-3 III Monochrome. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.