Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Nikon P300 and Panasonic GH3 side-by-side from the front, back and top in their relative dimensions. Nikon P300 has external dimensions of 103 x 58 x 32 mm (4.06 x 2.28 x 1.26″) and weighs 189 g (0.42 lb / 6.67 oz) (including batteries). Panasonic GH3 has external dimensions of 133 x 93 x 82 mm (5.24 x 3.66 x 3.23″) and weighs 550 g (1.21 lb / 19.40 oz) (including batteries).
Below you can see the front view size comparison of Nikon P300 and Panasonic GH3. Nikon P300 is clearly the smaller of the two cameras. Its body is 30mm narrower, 35mm shorter and 50mm thinner than Panasonic GH3. As you can tell, actually this is not a fair size comparison since Nikon P300 has a built-in lens but Panasonic GH3 doesn't. Once you put a lens on GH3, its thickness will increase significantly.
Here is the back view size comparison of Nikon P300 and Panasonic GH3.
Now lets look at the top view comparison of Nikon P300 and Panasonic GH3.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon P300 is significantly lighter (361g ) than the Panasonic GH3 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on GH3, the total kit will weigh much higher.
Thanks to the weather sealed body of GH3, you can use this camera with more confidence under conditions where there is risk of exposure to water and dust compared to P300.
LCD Screen Size and Features
Nikon P300 and Panasonic GH3's LCD screens has the same diagonal size of 3".
On the other hand, Panasonic GH3 sports a Fully Articulated screen
which you can change the angle and position of the screen almost freely, making it possible to shoot from waist or over the head levels, and especially very useful for shooting selfies and video. Nikon P300's LCD screen is a fixed type which provides almost no flexibility in shooting positions compared to GH3.