I have been messing around with growth regulators, and the internodes are definitely compressed, and the leaves are incredibly reduced, but the shape has changed drastically. I’m going to use a different compound next year to see if I get less distortion.
Leaf size and internode length are determined by dividing the amount of energy available to the roots by the number of primary and secondary buds available to expand, and moderated by the amount of sunlight available. The leaf will grow until it meets the light gathering requirements the plant needs. Since it can absorb more sunlight in a sunny exposure than in a shady location, leaves grow bigger in the shade. The intensity of sunlight varies according to the amount of atmosphere it passes through, so the closer to the horizon the sun is the less the intensity and conversely, mid-day sun is the most intensive. A plant that needs a lot of light, but is too sensitive to mid-day sun may get all the sun it needs with full morning and/or full afternoon sun as long as mid-day sun is blocked. Such sites, as in under a tree are unusual and limited for most people. Sunlight intensity also varies seasonally. In Detroit, JM can take full sun up to June 1st and after August 1st (see where the latittude of Detroit enters and leaves the darkest red on the chart below).
To reduce any part of the size of leaves and internodes, you need to limit what makes them the normal size. Internodes continue to lengthen as the leaves continue to expand, the natural process of providing enough distance between leaves so they don't shade each other. They should have as much sunlight as possible. The plants should not be fertilized until after the leaves have expanded to the plant's requirement. There needs to be more primary and secondary buds ready to expand and divide all the root-supplied energy among the larger number of leaves. You can defoliate once a year which increases ramification and leaf size. Removing all primaries at that time coverts more tertiary into secondary, bringing the foliage closer-in, but may be a trade-off that limits ramification. You can remove all the primaries as they expand in spring which switches some secondaries to primary and tertiary to secondary, increasing ramification. Generally, the bigger the original leaf size, the more yearly cycles of leaf reduction will be required to obtain whatever the maximum number of buds can be. This does not work on all Acer, for example A. platanoides, Norway Maple.
Another step is stopping the growth of an expanding bud. The primary buds wintered-over will have three buds. One big one between two smaller buds. The large center bud will grow into a twig with one big leaf at the tip and a pair of smaller ~40% of the length, and the two smaller buds will expand into threes leaves. One to three secondaries will expand into three leaves. When the middle leaf of the three is expanding, cut it off when the internodes of the lower pair are the size you want. The two leaves left will grow, but smaller than they would have and with shorter internodes.
It takes some years of these cycles to achieve really small leaves and corresponding internodes.
This is the vine maple I hit with a growth regulator.
Distorted leaves and drastically reduced internodes. I waited untill the nodes started to expand before I hit it with the growth regulator (a GA inhibitor) and that was probably too late. The first two internodes expanded as normal and then the distorted leaves and short internodes happened.
It has not elongated in about 3 weeks.
Also it is just one of my “mad science” trees and not really in any form of training.