The name already suggests that shrinkage refers to reduction of the size and volume of the pottery, and therefore it can be mistaken for contraction and expansion, making it seem like they’re rather similar. However, that’s not the case.
While the other two examples are quite natural and happen while the piece undergoes some changes in the temperature, cooling off and heating up, shrinkage happens only in the firing and drying processes.
What is even more important, is that it’s irreversible. While the majority of pieces exhibit some signs of shrinkage, a professional should be able to predict the outcome by choosing the right materials, clay and glaze beforehand, otherwise the piece will be deformed.