It is chancy to assume just because a tree has small leaves in the ground pre-collection, that it will translate directly to small leaves in a container. Sometimes it is the location, not the tree that is forcing small leaves. Location can have bad soil (too acidic, too basic, deep shadow, constant grazing by deer, etc.) Put that same small leafed specimen in a container with ample resources and it will likely revert back to large leaves.
This is not always the case, but more often than not, it is.
Good luck. I hope it is a genetic variant.