This is an interesting thread. I have kept track of the weight of the clay that I use and saw a clear decrease over time. I have noticed that my goals of throwing thin pieces with "ideal" forms often conflicts with what people want (thicker pieces that insulate and feel solid, and some bowls with flat internal bottoms). I have become less interested in impressing my potter friends and have no problem throwing a piece that the user wants. But, it took me a few years to be comfortable enough in my ability to throw what I want, that I am willing to make thicker or flatter bowls when the need arises. It just takes a couple heartbroken friends when their "beautiful bowl" is chipped to convince me to make sturdier ware.