The U.S. penny is, of course, only worth one cent. It is not difficult to exceed that value in the process of minting coins. The United States Mint, which is responsible for the design and production of all U.S. coinage, has to make periodic evaluations of the the process by which pennies are minted, or manufactured, with an eye toward ensuring that the cost per penny does not exceed the value of the coin. Consequently, over the years the composition of the penny has been modified to reflect the cost to the government of various metals used in the penny's manufacture. Amounts of copper, zinc, nickel and other metals are varied according to their respective costs. Copper being particularly unstable over the past decade -- with prices rising to such an extent that copper theft emerged as a serious problem across the country -- the amount used in the manufacture of pennies will vary.
One such instance of the cost of manufacturing pennies exceeding the value of the coin occurred in the early 1980s. In 1982 it was determined that the value of copper had risen to the point where it exceeded the one cent threshhold. Subsequently, the composition of the penny was modified to increase the zinc content while decreasing the amount of copper. That is why the mass of the penny feels, and is, different before and after 1982.
"If your Lincoln Memorial penny has a date before 1982, it is made of 95% copper. If the date is 1983 or later, it is made of 97.5% zinc and plated with a thin copper coating."
Zinc pennies weigh less, whereas copper pennies weigh more.