I just realized this yesterday, when we were trying to figure out how the PCs were going to retrieve a nice-sized treasure hall. A small sack holds 50 gp or 5 pounds (less than a grocery bag;) a large sack or pack holds 300 gp or 30 pounds. Thus, the large sack, when full, looks like a sack of potatoes; it's not quite as big as a sandbag. The small sack is more like how I imagined a large coin pouch. I'm guessing this is an artifact of the original equipment lists being "gold pieces only"; the cost for a small sack -- actually, a pouch -- should be about 3-4 sp; the 1 gp small sack should be about half the size of a large sack, maybe about the size of a bag of fast food, but sturdier.