Answer: Plants that are grown well will grow to their maximum potential and size, as you have discovered. A plant that is crowded and struggling for water, nutrients and light will not grow as well or as large as a plant grown without all that competition. Most plants, do though, end up within a predictable albeit rough size range. That is why you will usually see ultimate plant heights and widths expressed as a range rather than one single size.
It is your garden, so your preference for spacing is the one that counts. If you know they always end up looking crowded to you, then space them a bit further apart. Some gardeners, on the other hand, like them crowded together; other gardeners are impatient and want them to look "full" sooner. In any case, the spacing on the labels should be considered as a "guide" rather than a rule.
You might also consider checking some specialty reference books such as Perennials for American Gardens by Ruth Rogers Clausen and Nicolas H. Ekstrom for more detailed information about specific cultivars; sometimes the labels can be a bit vague.
Q&A Library Searching Tips