Below you can see the front-view size comparison of the Olympus E-M10 III and Olympus E-PL9. The Olympus E-PL9 is clearly the smaller of the two cameras. Its body is 5 mm narrower, 16 mm shorter and 11 mm thinner than the Olympus E-M10 III.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.