Below you can see the front-view size comparison of the Panasonic FZ2500 and Olympus E-M1. The Olympus E-M1 is clearly the smaller of the two cameras. Its body is 8 mm narrower, 8 mm shorter and 72 mm thinner than the Panasonic FZ2500.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.