I think unique coloration would work.
Personally, I have the opposite problem.
If the Matoran are short, they feel too much like children, and it makes them seem helpless. The Toa aren't around all the time (especially on a big island), it only makes sense for Matoran to be able to take care of themselves.
Actually, last I heard some G3 Matoran can use mask powers, it just doesn't come naturally to them like the Toa.
But, since it requires a lot of discipline and training, I don't think most Matoran would have masks of power.