Stoker ash is generally "chunky" so the volume is pretty high. You could probably crush a full bushel tub down to a few inches thick. The hand fired units generally burn it up better plus you have the crushing action of the grates so it's volume will not be as much. I've mentioned it before, the same coal that I burned in the Van Wert when burned in a Franco Belge came out as nothing but powdery ash. The Van Wert ashes would normally be chunky.
The time of the year will effect it too, at his time of the year it has lot longer to "stew" in the fir pot on the Van-Wert so you get a finer ash.