Answer: There are so many Japanese maple cultivars that it's difficult to know what you're growing without the benefit of seeing the leaves. However, the size leads me to believe you're growing Laceleaf or Dissectum Japanese maple. Laceleafs are characterized by two main traits: first, their leaf lobes are very narrow and serrated, giving them a lacy appearance. Second, nearly all are shrublike, forming
a weeping, mushroom-shaped mound that is seldom seen more than 5' tall. (They do get larger, but take many years to do so.) If this is what you're growing, no amount of pruning will coax it into becoming larger.
Best wishes with your tree!
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