Your question was answered on your duplicate post in the Japanese garden forum. The answer didn't change by moving it here :-)

That person suggested moving it here.

There are ways to prune a Japanese Maple so the inner structure of the branching can be seen but a tree of this size with this much foliage on it would be impossible to correctly prune without extensive understanding of technique. This will not offer much help of making the tree "smaller" anyway. I agree with Gardengal in moving some of the smaller plants in this bed and let this beautiful specimen be the centerpiece of this part of the landscape. The tree isn't too large for the bed but it sounds like it may be too large for your taste.

It's gorgeous just the way it is. If you want to prune it, YouTube has some very good tutorials with trees similar to yours.

I would prune the dead and dying branches that are growing under the outer ones. They're going to eventually get shaded out anyway. I'd cut them off where they originate on the trunk. This results in a thin umbrella with a wavy edge, not a bowl cut. Do not cut any branches at their tips. If the branch is 'too long' cut it off where it begins, not just short of where it's touching the ground. You want to show some of the curved trunks that is now hidden by leaves. That's one of the prime attributes of this plant and should be partially visible in the summer as well as the winter. I always say, "If ya got nice legs, show em'!"

Your maple should be center stage and the shrubs on the right in picture one should be moved to the wings.