The very short answer is that when coins were new, pennies (properly "one-cent coins") were made out of copper and dimes were made out of silver and the size of coins of different metals were not related to each other. Instead, early coins were made in similar sizes and shapes as other common currency at the time, including British, Spanish and French coins, but what's the fun in an answer that short?

By a 1792 law, United States minted coins had to be made of gold, silver or copper. The different coins were sized proportionally, but only to others of the same metal. A gold $5 coin was twice the size of a $2.50 coin and half the size of a $10 coin. Silver coins were all based around the silver dollar, which was the size and shape of the commonly used Spanish silver dollar. [U.S. Mint] Silver coins were the dollar, half dollar, quarter dollar, dime and half dime. The smallest silver coin, the half-dime, was half the size of our current dime and 1/20th the size of a dollar coin. The one-cent and half-cent coins were made out of copper and had their own scale. It just so happens that the 1/10th silver dollar coin, or the dime, was smaller than the one-cent copper coin.

The half-dime has a fairly short life in the American pocket. Over time it became apparent that the half-dime coin was just too small to be handled and used easily and in 1866 it was redesigned to be a larger coin. Silver was removed from the alloy and replaced with...can you guess? Nickel!

The coin with the shortest run was the 20-cent piece, which was minted for 4 years. Of those 4, only 2 years were dedicated to coins put into circulation.

Since the very first US minted coins (11,178 copper cents minted in March of 1793) coins have changed some in size, design and metal content but for the most part they have stayed very close to the originals. Our coins are the size they are because the foreign and British coins our ancestors were used to handling were the sizes they were! Similarly, the way coins are printed, with the back upside down compared to the front is simply a matter of tradition. Even the US Mint can't say why it's done other than because that's how it was always done!

For more interesting facts about coins and money, check out the US mint www.usmint.com website and that of the US treasury! www.ustreas.gov