Nickels on the other hand, were designed with weight in mind. They were designed as 5 grams in the metric units when they were introduced in 1866 (wikipedia) as a copper-nickel alloy. Thus they happen to weight about twice as much as the dime even today.

The Answer:

Actually, the first five-cent coin in U.S. history was made of silver and was smaller than today’s dime.

That’s because when coins were first produced by the U.S. Mint in 1793 the U.S. standard coin was the silver dollar, and additional coin denominations were made with a proportionate silver content to the dollar. This, in turn, established the size of each. For example a ten-cent coin, or dime, contained 1/10 the silver found in a dollar.

The five-cent coin (which contained 1/20 the silver found in a dollar) was eventually determined to be too small to handle, and the five-cent coin we know today as a “nickel” was created in 1866. The size of the coin was increased and its metallic content was changed from silver and copper to a combination of copper and nickel.

At one time, the dime was made of a more precious metal, silver, than a nickel was. We haven’t had silver dimes since 1964. The size retention is from tradition, and from the vending machine industry lobby.