Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Pentax P80 and Panasonic GX1 side-by-side from the front, back and top in their relative dimensions. Pentax P80 has external dimensions of 102 x 59 x 25 mm (4.02 x 2.32 x 0.98″) and weighs 125 g (0.28 lb / 4.41 oz) (including batteries). Panasonic GX1 has external dimensions of 116 x 68 x 39 mm (4.57 x 2.68 x 1.54″) and weighs 318 g (0.70 lb / 11.22 oz) (including batteries).
Below you can see the front-view size comparison of the Pentax P80 and the Panasonic GX1. Pentax P80 is clearly the smaller of the two cameras. Its body is 14mm narrower, 9mm shorter and 14mm thinner than Panasonic GX1. As you can tell, actually this is not a fair size comparison since Pentax P80 has a built-in lens but Panasonic GX1 doesn't. Once you put a lens on GX1, its thickness will increase significantly.
Here is the back view size comparison of the Pentax P80 and Panasonic GX1.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Pentax P80 is significantly lighter (193g ) than the Panasonic GX1 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 GX1, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Pentax Optio P80 nor Panasonic Lumix DMC-GX1 provides any type of weather sealing in their bodies, 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 Compact Cameras with Weather Sealing Top Mirrorless Cameras with Weather Sealing
LCD Screen Size and Features
Panasonic GX1's 3.00" LCD screen is slightly larger than Pentax P80's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.