On earth they readily convert. The size (historically) was determined by the volume needed to get that much mass. At one time payment was measured in weight as you could never trust the other person (or the person who fooled them) to not mix some less valuable metal on with their coins. Those days are mostly gone but the history is weight (which on earth is the same as mass) is important not volume.
> You could never trust the other person (or the person who fooled them) to not mix some less valuable metal on with their coins
Another major problem was people taking shavings from the edge of coins and trying to pass them off as unaltered; it's the reason many coins have some kind of pattern imprinted around the edge.
Hence the partially empty cereal boxes and snack bags which sell by weight but package to maximize visual perceived volume. I still recall the disappointment as a kid when I got a new box of cereal only to see that a huge portion was "already gone".
