Below you can see the front-view size comparison of the Panasonic GH4 and Panasonic FZ2500. The Panasonic GH4 is clearly the smaller of the two cameras. Its body is 5mm narrower, 9 mm shorter and 51 mm thinner than the Panasonic FZ2500.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.