Plant Life: Living Means Getting Larger
People often ask "how large does this plant get?" Its a perfectly reasonable question with a very complex answer. What makes me crazy is when plant marketers pretend that there is an easy answer. You see this mostly with trees and larger shrubs, where the label assures us that it will only get so many feet high. People imagine that they can plant it under a wire, window, or whatever and it will magically stay the perfect proportion and they will never have to prune it.
The only problem is that it's not true. The reality is that all plants are indeterminate, which means that as long as they are living, they are growing. True annuals (unlike tender plants that we grow as annuals) will stop growing at the end of the season after they set seed because they die (tender plants stop growing because they die of the cold). Most perennials will keep growing by spreading sideways - becoming bigger clumps or sending runners out away from the main plants. I have found that many perennials will grow taller every year as well as making bigger clumps. I have four foot tall balloon flowers (platycodon grandifloras), which most catalogs claim top out at less than two feet.
Shrubs often do seem to top out at a particular height, but that is because their wood cannot always hold several years growth upright and the shrub begins to sprawl. It's fine if its mass doesn't become a physical or design problem, but most plants' actual volume increases more and more each year, and its mass can sneak up on you.
Trees rarely stop growing taller, but half foot of growth is a lot more noticeable on a five foot sapling than on a twenty-five foot tree. Also, many ornamental trees' wood is not great at supporting several years worth of growth, so they start to spread like shrubs do. This can be very attractive (I especially love enormous japanese maples), but it completely changes the proportions of you garden. So you either have to live with odd proportions, redesign everything to go with the short but sprawling tree, or prune it - which what you were trying to avoid in the first place.