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It is forgotten in our modern age of fiat currencies, but money was historically not just a representation of value, but a physical store of value in itself. Coins contained a certain amount of precious metal, particularly silver or gold.
The size of the coin is only indirectly related to its value, because the true value of a coin lay in its total silver or gold content. A small coin with a high silver content would have been more valuable than a large coin that was mostly copper, which incidentally is why the U.S. dime (10ยข) is traditionally smaller than the penny (1ยข).
Expanding the money supply required increasing the number of coins in circulation, but the temptation to reduce the silver or gold content of the coins was irresistible to rulers. It reduces the precious metals in circulation to the benefit of the ruler, and contributes to inflation (which is, after all, simply the devaluation of money) which would help a ruler pay off debts.
As Doug Smith notes, the follis was debased several times, and was far from the only Roman coin which suffered that fate.