sorry thats so incorrect, micro baloons are fine glass particles formed into spheres, used for adding volume to adhesives and resin.

its used for increasing the volume of the adhesive to reduce the weight of the mixture without reducing the strength, caution should be used when sanding the adhesive/micro baloons, since the mico baloons sand into glass particles, a dust mask should be used at all times when handleing the stuff,before,durring, and after use, it doesn't break down when inhaled.

talc is finely ground gypsum, and absorbes the adhesive, adding volume without reducing the overall weight of the adhesive,it also adds no strength to the resin.

milled fiber is just stranded fiberglass which also absorbes resin adding strength but not reducing the weight of the mix like pilotfighter pointed out.