The Dime Is The Only Coin We Really Need

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It is a holdover from when coins were roughly based on the value of the metals involved. The dime was silver and weighed 1/5th that of the silver half dollar etc. The Half Dime weighed half that of the Dime.

The Penny, Half Cent and 2-cent coins had relative weights of copper.

The Nickel was a loner in that there was no other coin in its class except for the short-lived 3-cent piece. Interestingly there was also a silver 3-cent coin that weighed proportionally to that of a dime.

If the coins had historically been of the same class, in the same metals, then the weights would have been proportional. Since weight is based on size when made of the same materials the 5-cent coin would be smaller than the 10-cent one.