The same things that reduce leaf size on any Maples wok in Red Maple:
Defoliation is OK, but is done too early they will grow back to normal size before the season ends..
Mostly-
Reducing the size of the pot it is growing in really seems to have an immediate, dramatic effect.
That coupled with pruning to create ramification leads to limited roots supplying a bunch of leaves and that creates smaller leaves!
The "problem" with these guys- you simply cannot reduce the size of the STEMS. not just regarding internode distance, but the diameter of even the smallest twigs is dramatically larger than a JM or Trident! No amount of leaf reduction changes that dramatically and thick stems= poor twig ramification. On top of that, the Red Mapkes simp,y do not ratify as easily as a JM air Trident. Hard cut backs to short nodes leads to twig/ branch death sometimes which sends you right back to the drawing board with branch development. Trying to make a nice, twiggy ramified canopy on these guys like you see on other Mapkes is fitting a square peg in a round hole... Just my observations after a couple years of working with them in all ages from seedlings up to large, old well developed Bonsai and pre-bonsai.