Thickening sauces and soups can be accomplished by a variety of ingredients but most rely on a starch. Even though starches such as flour and potatoes are mostly carbohydrate, they also have proteins that, when heated, interact with liquid to thicken them. Most cream soups begin with a roux and milk. With chowder, it is primarily the potatoes that act as the thickening agent.
I prefer russets or long whites because of the higher starch content. These potatoes also don't hold together as well when heated and, as they breakdown, make for a thicker soup.