Below you can see the front-view size comparison of the Panasonic GH5S and Panasonic GX8. The Panasonic GX8 is clearly the smaller of the two cameras. Its body is 6 mm narrower, 20 mm shorter and 24 mm thinner than the Panasonic GH5S.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.