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 RZ18 and Pentax Q-S1 side-by-side from the front, back and top in their relative dimensions. 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). Pentax Q-S1 has external dimensions of 105 x 58 x 34 mm (4.13 x 2.28 x 1.34″) and weighs 203 g (0.45 lb / 7.16 oz) (including batteries).
Below you can see the front-view size comparison of the Pentax RZ18 and the Pentax Q-S1. Pentax RZ18 is 8mm narrower and 1mm thinner than Pentax Q-S1 but it is also 3mm taller. As you can tell, actually this is not a fair size comparison since Pentax RZ18 has a built-in lens but Pentax Q-S1 doesn't. Once you put a lens on Q-S1, its thickness will increase significantly.
Here is the back view size comparison of the Pentax RZ18 and Pentax Q-S1.
Now lets look at the top view comparison of Pentax RZ18 and Pentax Q-S1.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Pentax RZ18 is 25g lighter than the Pentax Q-S1 but we don't think this will make a significant difference.
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 Q-S1, the total kit will weigh much higher.
Weather Sealing
With its weather-sealed body, we believe that the RZ18 has a big advantage over the Q-S1 which doesn't provide any kind of environmental sealing.
LCD Screen Size and Features
Pentax RZ18 and Pentax Q-S1's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.