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 S1 and Pentax Q-S1 side-by-side from the front, back and top in their relative dimensions. Pentax S1 has external dimensions of 114 x 58 x 28 mm (4.49 x 2.28 x 1.1″) and weighs 157 g (0.35 lb / 5.54 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 S1 and the Pentax Q-S1. As you can tell, actually this is not a fair size comparison since Pentax S1 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 S1 and Pentax Q-S1.
Now lets look at the top view comparison of Pentax S1 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 S1 is significantly lighter (46g ) than the Pentax Q-S1 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 Q-S1, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Pentax Optio S1 nor Pentax Q-S1 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
Pentax Q-S1's 3.00" LCD screen is slightly larger than Pentax S1's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.