Hi Kris,
I just found a metal bobbin to measure and they are slightly larger in outside diameter than the plastic version by about 0.010".
The metal bobbin materal is thinner therefore it appears that more line could be wound on it; but it would be more difficult to reduce the diameter of the metal bobbin vs the plastic version to allow them to fit to the bottom the matchcase.
The metal bobbins were made by; Prym-Dritz Corp (
www.dritz.com ) of Spartanburg SC, they are called Class 66 Drop-in Bobbins and I bought a pack of 4 at Wal-mart for a couple dollars.
Standard Stuff: I have no connection to the above company, other than there bobbins are in my PSK!
Mike