It's not totally clear from the article, but it would seem that the small coins are worth much less than the large ones since there is little hardship involved

It's not totally clear from the article, but it would seem that the small coins are worth much less than the large ones since there is little hardship involved in their excavation and transportation. So the small ones function more like coins, and the large ones are more like valuable property which can be exchanged via an oral deed.