AusBonsai

I have had this little hokkaido elm for a few years, I don't think it has grown much at all. I am thinking about putting it in the ground for a few years.
Today I noticed some surprising new growth. The leaves on the new branch are bigger than most of the sub-branches.

Geez on the thumbnail i thought that was an elm grafted onto a juniper. Massive difference with the leaves.

This is known as "reverting back". Basically, this form came about when somebody noticed an odd compact growth (known as a sport) on a normal Ulmus parvifolia. This growth became a cultivar when people started taking cuttings and growing it deliberately. Occasionally, cultivars from sports may return to their original state. That is what is happening here. In most cases, the original form is supperior and the rest of the plant will die if it is left to grow. This is the case with elms. So, if you want a regular chinese elm, you can let it grow. If you want the hokkaido, then prune this growth off.

Joel

It's a Triffid! Cut it off!!!

I agree with Joel. Quite a few trees/shrubs can revert back to the original. In this case it's the size of the leaves, but also Variegated ones can revert back to the original plain green leaves.

And cut it off NOW!!! That little shoot will be sucking all the growth from your tree.

Stick the cutting into an open sandy mix and get yourself a new Chinese Elm to play with.