Why Are Dimes Smaller Than Nickels

Size was based on the value of the metals...

When the United States was on a gold standard coins were worth the amount of metal in them. The nickel is worth 5 cents worth of nickel the material out of which it is made and the dime was worth ten cents worth of silver the material out of which it was made. Obviously, nickel is not as valuable as silver so it is larger than it's 10 cent counterpart. The penny was worth 1 cents worth of copper and the quarter was worth 25 cents worth of silver.

Obviously we aren't on a metal standard anymore but the coins retain their size by tradition.

10 cents silver dimes lasted into the 1960s (1964-US and 1968-Canada) when the use of vending machines was widespread. I suspect they did not change the size of the dime to avoid inconveniencing vending machine operators with an expensive coin mechanism upgrade. In fact post-1964 US coins (10 cents, 25 cents) have the famous cupronickel and copper sandwich structure to simulate the electronic signature of silver coins. Canadian vending machines had only minor changes made to them to accept both nickel and silver coins. Today the mint has no qualms about changing dollar bill images often (to thwart counterfeiters), but also causing much frustration to vending machine operators.

Another secondary reason could have been not to confuse the blind. The dime can be felt out really easily with its thin size and reeded edge and is not confused with the penny. If the mint came up with a new 10 cent coin with a size between a 5 cent and 25 cent coin it would be confusing to the blind. Do you have two non-reeded coins (5 and 10 cents) of about the same size or two reeded coins (10 and 25 cents) of about the same size ?

What a wild question about the dime being smaller than the nickel. I never thought about it UNTIL NOW. And to Dave H -- thank you. I will spend much of my day wondering how I missed this.