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-m and Pentax S1 side-by-side from the front, back and top in their relative dimensions. Pentax K-m has external dimensions of 123 x 92 x 68 mm (4.84 x 3.62 x 2.68″) and weighs 590 g (1.30 lb / 20.81 oz) (including batteries). 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).
Below you can see the front-view size comparison of the Pentax K-m and the Pentax S1. Pentax S1 is clearly the smaller of the two cameras. Its body is 9mm narrower, 34mm shorter and 40mm thinner than Pentax K-m. As you can tell, actually this is not a fair size comparison since Pentax S1 has a built-in lens but Pentax K-m doesn't. Once you put a lens on K-m, its thickness will increase significantly.
Here is the back view size comparison of the Pentax K-m and Pentax S1.
Now lets look at the top view comparison of Pentax K-m and Pentax 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 (433g ) than the Pentax K-m 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-m, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Pentax K-m nor Pentax Optio 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 DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Pentax K-m and Pentax S1's LCD screens have the same diagonal size of 2.70 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.