I've never heard of a volunteer maple, but I do grow maples, Japanese mountain maple, trident maple, and English field maple as bonsai and in the garden.
Is the main trunk 6 foot long and growing foliage at the top, with two branches low down that are growing? If so, one of the best ways to increase trunk size is to cut the main trunk about an inch above the topmost of those two branches, the lower one, cut it back to about an inch above the lowest leaves. This will produce more vigorous growth in the upper branch that will become the main trunk, it will also spur the lower branch to start producing more growth.
Important bit, if the tree is grafted and the two lower branches are growing from below the graft, what you have is the stock plant taking over the tree, then, the important thing is to maintain the health of the main trunk and lop off the suckers.
I'm in the middle of preparing a hub about growing maple in the ground to develop a thick trunk for bonsai.