Dimes were originally made from silver beginning in 1796.
They were smaller than the quarter- and half-dollars, as well as the dollars.
Five-cent pieces were minted only starting in 1866, and were made larger, with a plain edge, and from a copper-nickel composite to distinguish them from the smaller dimes.
In 1965, dimes (along with quarters and half dollars) began to be minted using the same copper-nickel composite, rather than silver. However, dimes still maintain the small size from tradition.