Below you can see the front-view size comparison of the Fujifilm X-T3 and Olympus E-M5 III. The Olympus E-M5 III is clearly the smaller of the two cameras. Its body is 8 mm narrower, 8 mm shorter and 9 mm thinner than the Fujifilm X-T3.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.