The good news is that there is a way to decrease leaf size in a Scheflera. The bad news is that you probably won't like the way to do it......or the long-term result.
If you defoliate the entire tree, it will grow back more abundant, and smaller leaves. Also this will encourage branching in the tree.
The part you won't like is the look of the tree for the weeks it takes to re-grow the leaves and the fact that this result is only temporary.
Over time the leaves will grow larger again as you've seen in your tree currently.
There is a Schef. that will maintain smaller leaves. The variety is S.aboricola. I have several and I'll be honest with you. It's a crap shoot as to which of even this variety will maintain smaller leaves. All of mine come from 'arboricola' seeds but only about 25% still maintain small leaves.
I was inspired to buy the seeds when I saw a Schef. arboricola growing in a pot in the lobby of a school. It was not trained as Bonsai per se but looked like a tree. I will also say that it was about 36" tall with a 36" spread. I think any Schef. will need to be that size to look somewhat in proportion due to leaf size.
This is just my experience. Others may vary.
"As the twig is bent, so the tree inclines"
"I rarely agree with most of what I say........." -
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