rockm

You shouldn't be worried about large leaves. The tree needs to grow some. Large leaves are a response to lower light conditions, more root run, etc. They aren't a bad thing necessarily. If you're planning on letting this tree get some heft in the trunk, don't obsess about bonsai care now. Just let it go. You actually WANT big leaves and robust growth. I would not trunk chop this tree for another five years (or more).

I would let it develop a bigger root systems also. Korean hornbeam have weaker root systems. They need alot of time to develop a strong root system. Larger leaves and new growth indicates the plant is pushing new roots to support that growth.

Don't be so anxious about development. This tree could take some time. They are worth the trouble, as they make great bonsai subjects.