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