Looks like it needs a lot more light than it is getting there, move it to a sunnier window. When mature, Radermachera sinica can get to 10-15m or more tall. It obviously won't do this easily indoors, but it will try!
Michael is correct. It needs more light than it is currently receiving. One can tell this by the wide spacing between the foliage. This is a sign that it is trying to reach for more light. Anytime you are attempting to grow woody plants indoors, it is good practice to (1) prune regularly to induce more branching and slow vertical growth and (2) make efforts to place it in a high light environment. When the weather warms, place it outdoors, but pay close attention to cold overnight temperatures and to start in a shady/partly shady spot before introducing it to full sun exposure. Although you may not consider your plant to be a "classic" bonsai tree, it is a tree in a pot, and you can read up on all the bonsai techniques that may apply to your specific tree. I have used many different bonsai techniques with my own indoor/outdoor container trees.
