Microballoons can reduce the weight added for a volume of epoxy used to fill a gap by over 60%. IT all depends on how much microballoons vs how much epoxy to make up the mix.
I also have no experience with StyroSpray, but when I want the lightest microballoon filler possible I mix the epoxy, paint the area to be filled with some then mix in the microballoons. This ensures the surface on the structure has adequate epoxy for the stiff paste filler to stick. Otherwise you can mix in enough microballoons that the resulting filler won't stick to the model.