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 K-x and Pentax RZ18 side-by-side from the front, back and top in their relative dimensions. Pentax K-x has external dimensions of 123 x 92 x 68 mm (4.84 x 3.62 x 2.68″) and weighs 580 g (1.28 lb / 20.46 oz) (including batteries). Pentax RZ18 has external dimensions of 97 x 61 x 33 mm (3.82 x 2.4 x 1.3″) and weighs 178 g (0.39 lb / 6.28 oz) (including batteries).
Below you can see the front-view size comparison of the Pentax K-x and the Pentax RZ18. Pentax RZ18 is clearly the smaller of the two cameras. Its body is 26mm narrower, 31mm shorter and 35mm thinner than Pentax K-x. As you can tell, actually this is not a fair size comparison since Pentax RZ18 has a built-in lens but Pentax K-x doesn't. Once you put a lens on K-x, its thickness will increase significantly.
Here is the back view size comparison of the Pentax K-x and Pentax RZ18.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Pentax RZ18 is significantly lighter (402g ) than the Pentax K-x 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 K-x, the total kit will weigh much higher.
Weather Sealing
Thanks to the weather-sealed body of the RZ18, you can use this camera with more confidence under conditions where there is risk of exposure to water and dust compared to the K-x.
LCD Screen Size and Features
Pentax RZ18's 3.00" LCD screen is slightly larger than Pentax K-x's 2.7 screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.