Below you can see the front-view size comparison of the Fujifilm S5 Pro and Panasonic GX8. The Panasonic GX8 is clearly the smaller of the two cameras. Its body is 14 mm narrower, 35 mm shorter and 11 mm thinner than the Fujifilm S5 Pro.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.