You can consider the thickness to be the ratio of dissolved starches to water. The more starch, the thicker the sauce. The less water, the thicker the sauce.
0 dissolved starch / 2 liter water = 0 thickness
Just reducing the amount of water by half doesn't fix the problem.
0 dissolved starch / 1 liter water = 0 thickness
Starches can be found in the ingredients of the soup/stew. For instance, if you cook the soup long enough, the noodles will start to dissolve into the broth, making it thicker. Of course, if you are making chicken NOODLE soup, this probably isn't the desired result.
One option would be to add one batch of noodles at the start of the cooking process. Wait till they dissolve and the soup is nearly done before adding the rest of the noodles.
Another option is use a different starch food ingredient like some chopped up potato or navy beans at the beginning, and then adding the noodles near the end again.
Both of these options require a long cooking time, but in my opinion, add nutritional value to the soup.
There are also fast starches like plain flour, or corn starch that can work in anywhere from a few minutes to a few seconds. The easiest way I've found to add them is to mix them with a little bit of cold water to form a slurry, and then pour the slurry into the boiling soup while stirring rapidly. Using the right amount takes some practice, but remember that they both thicken a bit more as the soup cools, so don't add too much when it's boiling.
For sauces, I like to reduce the amount of water a bit first through boiling before adding a quick starch. For stews and soups I prefer a nutritional starch at the beginning of the cooking process.