Answer: There are many many different kinds of Japanese maples; some are simply more vigorous than others and some are simply more robust-looking than others. In addition, some of the named selections are potentially absolutely spectacular while others are simply beautiful. A dwarf plant will grow proportionately more slowly than a non-dwarf, but that is about the only generalization one can make without knowing which types you are seeing.
To some extent initial plant quality and condition during the production and retail phases will dictate future years' growth; good early care during the establishment phases is critical, too. (Regular attention to watering and a generous layer of mulch are very helpful.) After that, general growing conditions will determine the ultimate success of the plant, so selection of the planting spot is quite important. Japanese maples do best in moist yet well drained soil with plenty of organic matter; they do not do well in a windy spot or in a baking hot spot in full sun with reflected heat.
If you suspect that your soil is poor, you might wish to perform some basic soil tests. Your County Extension (784-1001) can help you with the tests and with interpreting the results -- it is important to test in order to determine which amendments (if any) are needed.
Good luck with your tree!
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