Below you can see the front-view size comparison of the Fujifilm X-A3 and Fujifilm X-T20. The Fujifilm X-A3 is clearly the smaller of the two cameras. Its body is 1mm narrower, 16 mm shorter and 1 mm thinner than the Fujifilm X-T20.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.