They do stay modest in size as long as they are in a container. They grow much faster in the ground so I wonder what would happen if you re-potted it in progressively larger pots more often than one would normally do. Try potting up in early spring and as soon as the roots fill the pot do it again and so on.
I've never tried this myself but I had one in a container for 20 years and as soon as I moved to an area where it could be in the ground I planted it and it really took off.
