I planted a clementine or mandarin tree from seed maybe two or three years ago. I don't think it has grown much over the last couple of years. See photo (with a sticky note for scale). It doesn't grow, but it also stays alive. I planted another one around the same time that, although not huge itself, it much bigger than this one (see other photo—any advice for this one is also appreciated). I live in the Atlanta area and recently put the tree outside. What can I try to get it to get bigger?
Second tree for reference: