Answer: It is possible to prune too severely and end up with dead or terribly misshapen shrubs. Since you've already pruned them down by a third and are keeping them at that height, you may be able to gradually reduce their size even more by pruning twice a year instead of once a year. Pruning encourages new growth so a once-over with shears in the spring and again in mid-summer should allow you to slowly reduce the size of the plants while still keeping them full and dense. I'd avoid cutting them down more than a third in any one pruning session.
Best wishes with your project!
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