The company website says the Ca crystals form a scaffold for "fibroblast ingrowth". So I am assuming the purpose of micronizing the particles (which could be a misnomer based on what you say) is so they don't last too long - in effect leaving collagen deposited by fibroblasts in the injected area after the scaffold is resorbed
PS: you hear about drugs that are "micronized" - usually to help gut absorption - but i wonder if the term is used pretty loosely in general