The smaller media mills more efficiently (when each ball is heavy-enough to give the desired impacts), not because of the smaller space, but because there are more contact-points in a mass of small balls than in a mass of large ones.
Smaller media is most effective, as long as it's heavy enough to break whatever you are milling. There is something of a relationship where larger media is more effective at larger particles of the same material and as the size is reduced, you gain efficiency with smaller media.
Pretty much, there is a optimum size for everything.
At least that is my understanding. Again, i have been wrong before.
I'm using 10mm lead balls cast from mainly wheel-weights, with some added tin scraps, and water quenched when cast. Not really compared the media to anything else, but it works.