Iron ore and stone are often very dense materials, especially when compared to coal. Most wagons were designed to accomodate a volume of coal equating to the wagons' full load rating.
For example a full 7 or 8 plank open wagon would contain 12 tons of coal, while a 12-ton capacity wagon built for stone traffic would often have lower 3 high sides.
Typically then a full load, by weight, of iron ore might well form just a small heap on the wagon floor, ranging from barely enough to cover the entire floor area to just a couple of planks in depth (maybe 6-9inches), depending on the density of the ore being conveyed.