Both Pentax 645Z and Pentax 645Z have Medium format sized 51.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Pentax 645Z and Pentax 645Z 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 645Z and 645Z sensor size comparison.
Pentax 645Z and Pentax 645Z have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.