Below you can see the front-view size comparison of the Fujifilm GFX 50R and Panasonic GX8. The Panasonic GX8 is clearly the smaller of the two cameras. Its body is 28 mm narrower, 19 mm shorter and 3 mm thinner than the Fujifilm GFX 50R.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.