Below you can see the front-view size comparison of the Fujifilm GFX 50S and Pentax 645Z. The Fujifilm GFX 50S is clearly the smaller of the two cameras. Its body is 8mm narrower, 23 mm shorter and 32 mm thinner than the Pentax 645Z.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.