My understanding (I may be wrong so someone correct me if I am!) is that it doesn't have to do with the frequency but the actual volume when you go. If you go a little, that's small volume. If you go a lot or find yourself thinking, "Hmmm, I better flush before I continue..." then that's large volume
I've seen studies where they say, "High stool frequency" or "High volume of stool" but I've never interpreted those to be interchangeable.