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 Panasonic GX850 and Panasonic ZS8 side-by-side from the front, back and top in their relative dimensions. Panasonic GX850 has external dimensions of 107 x 65 x 33 mm (4.21 x 2.56 x 1.3″) and weighs 269 g (0.59 lb / 9.49 oz) (including batteries). Panasonic ZS8 has external dimensions of 105 x 58 x 33 mm (4.13 x 2.28 x 1.3″) and weighs 210 g (0.46 lb / 7.41 oz) (including batteries).
Below you can see the front view size comparison of Panasonic GX850 and Panasonic ZS8. Panasonic ZS8 is 2mm narrower and 7mm shorter than Panasonic GX850 but they have the same thickness of 33mm. As you can tell, actually this is not a fair size comparison since Panasonic ZS8 has a built-in lens but Panasonic GX850 doesn't. Once you put a lens on GX850, its thickness will increase significantly.
Here is the back view size comparison of Panasonic GX850 and Panasonic ZS8.
Now lets look at the top view comparison of Panasonic GX850 and Panasonic ZS8.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Panasonic ZS8 is significantly lighter (59g ) than the Panasonic GX850 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 GX850, the total kit will weigh much higher.
Unfortunately neither Panasonic Lumix DMC-GX850 (Lumix DMC-GX800 / Lumix DMC-GF9) nor Panasonic Lumix DMC-ZS8 (Lumix DMC-TZ18) provides any type of weather sealing in their body, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top Mirrorless Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Panasonic GX850 and Panasonic ZS8's LCD screens has the same diagonal size of 3.00".
On the other hand, Panasonic GX850 features a Tilting screen
where you can change the angle of the screen to make it easier to shoot from waist or over the head levels whereas the Panasonic ZS8 has a fixed type screen which provides less flexibility in shooting positions compared to GX850.