I know that they are generally speaking different sizes so that blind people will be able to handle change (which is also why in some countries bills are different sizes, depending on denomination, so god knows why they don't do it in the US). Nickel (or more accurately, whatever nickel alloy they use to make nickels) is probably cheaper than whatever silver alloy they use to make dimes. That's presuming they still use silver to make dimes, which I don't think they do. Pennies are especially weird because these day's their basically worthless, cause more than a penny to make, therefore costing the US mint money. But it's got Lincoln on it, so the state of Illinois (among others) tends to put up a fight at the suggestion of eliminating them. True story.