Below you can see the front-view size comparison of the Panasonic FZ1000 and Panasonic FZ300. The Panasonic FZ300 is clearly the smaller of the two cameras. Its body is 5 mm narrower, 7 mm shorter and 14 mm thinner than the Panasonic FZ1000.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.