Below you can see the front-view size comparison of the Olympus E-PM1 and Olympus E-PL8. The Olympus E-PM1 is clearly the smaller of the two cameras. Its body is 5mm narrower, 3 mm shorter and 4 mm thinner than the Olympus E-PL8.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.