Haha. The smaller size provides more surface area for colonization of bacteria. It also packs tighter, so you would need less to create a DSB for denitrifying bacteria. Also, since it packs tighter, there is less of a chance detritus can settle in it. Plus the size enables you to keep certain species of inverts and fish (fungia plate, sand sifting gobies, etc.)
Larger size substrates are for the most part the opposite of these reasons.
You should have picked up Calfo's book! He has a whole section on substrates. Haha :lol2: