Because they are in need of different culture requirements. Imagine a 7cm tree in 40+ temp being treated the same as a 1 metre tree.
Try getting 5 or 6 layers of branching in that size tree, then pruning it with a pair of 20 cm scissors or taking a branch off with 20 cm branch cutter.
About the only thing that is the same is there need to be fertilised and watered the same, although when it is hot just a bit more. With the exception try putting a slow release fertiliser basket on that 7 cm tree or even some DL pellets.
Leave the re potting for two years then say good bye to the tree.
But as you see no different than growing a 1 metre tree.
Cheers
