Susan has been writing about massing as a way to make a garden seem substantial. There is another way, but you have to be able to resist temptation, which I often cannot: Put down the god-damned shovel and stop moving the perennials around on every whim!
Digging up and dividing them keeps them in a state of constant adolescence.
But if you let them be, some of them will grow to the size of Priuses, and you won't necessarily need to group them in multiples and plant them in drifts in order to achieve a sense of purpose in your garden. You can use other design strategies, such as repetition, which I find harmonious and pleasing and possibly better suited to a small city yard.
After four or five years in a good spot, perennials achieve something like stature, even the little ones like low-growing geraniums and campanulas. I can't say that they do this too much faster than a shrub.
Patience, however, is not much emphasized in the literature of flower gardens–where the operative lie is that perennials that don't flower in their first year after planting usually flower in their second, as if the deal is finished then.
Of course, there are exceptions. Larry Hodgson's excellent 2005 book Making the Most of Shade, which offers sprightly and unusually frank descriptions of several hundred pages' worth of plants, is constantly cautioning that the plants will become really impressive…if you don't fuss with them.
Here is his advice, for example, on aruncus:
Just plant it and leave it. This plant never needs dividing and thrives on neglect–moving it around or dividing it regularly slows it down.
Fortunately, my arunus dioicus are seeding themselves.
But I have loads of other substantial perennials…and empty spots waiting for a piece of them. Leave them alone is great advice, if you are capable of following it. Which, I am often not.