The nickel had a growth spurt....
The first five-cent pieces were small. Called "half dimes," they weighed exactly half as much as a dime because their values were based on the amount of silver used to make them. The half dime's tiny size (about 16 mm) meant the coin was hard to handle and easy to lose. In 1865, Mint Director James Pollock thought that a five-cent coin made of nickel alloy would be a good trade for the five-cent paper notes that were circulating then. It turned out to be a good replacement for the half dime, too!