Shell in garden snail
The snails enlarge the shell by adding material at the lip.  The addition of material is sometimes noncontinuous and shows up as rings, sort of like tree rings.  You can see the growth rings in this photo.


I found the photo here http://ediblesouthflorida.ediblecommunities.com/things-do/snails-slugs-millipedes-oh-my

As the shell spirals outward the diameter of the added rings gets larger and larger.  It has to get larger if it is to spiral - addition of the same sized rings would make a donut-like torus.