Below you can see the front-view size comparison of the Olympus E-M1 II and Panasonic GX8. The Panasonic GX8 is clearly the smaller of the two cameras. Its body is 1 mm narrower, 13 mm shorter and 4 mm thinner than the Olympus E-M1 II.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.