We have a dwarf orange tree and a full size Meyer Lemon tree. In the past six months we noticed both trees shrunk at least a couple inches. We know this because they are both up against a fence that is not shrinking. We suspect gophers, but both trees were planted in full gopher cages or wire, but we do not see any signs of gopher activity.
Shrinking Orange and Lemon Trees
Certified GKH Gardening Expert
It could be that the soil is fertile, and extremely well aerated. This would cause them to shrink some, as the soil compacts itself down over time. It will not hurt your trees. Here is a link to several articles for citrus care: https://www.gardeningknowhow.com/edible/fruits/citrus/