Most people start with medium sized plants and then grow them like they normally would but then trim and wire the plants into the forms that they want. After the trees are formed on top then they are constantly pruned but each time they are repotted (note it is still in standard nursery pots) the rootball is pruned back ever so slightly reducing it up to one-third of its original size. A good indepth google search will give you a little more detail thought

-J.P.