As 'vhs' mentioned, the weights were used on a scale--as a counterbalance to what is being weighed; not as a cup to fill.
Finding the two cups that most closely balanced the material being weighed--like gold dust and nuggets--would give a close approximation to the actual weight of the material on the other side of the scale.
The smallest cup would be covered by a cap, called a pill--one of the first objects in the collection to get lost. Finding the entire 'collection' ('pill' weights and 'case') can produce full value; anything less and the value is halved--or worse.
Nuremberg Germany produced many of these nested weights--and they were heavily replicated.
Don...