I think it's historical: The value of a coin was once a function of the material it was made from and the size of the coin. The lowest value coins were made fro

I think it's historical: The value of a coin was once a function of the material it was made from and the size of the coin. The lowest value coins were made from bronze; the next lowest were made from silver; and the most valuable were made from gold. Among coins made from the same metal, those which were bigger were more valuable. This explains the size and look of different British coins.