It has been noted that John McCain's obsession with earmarks is "bizarre," considering such spending only constitutes "18 billion out of a much larger pie [a federal budget of, gulp, $3 trillion]". As a matter of fiscal policy, the argument is correct. Earmarks, otherwise known as pork, just aren't a big part of total government spending.
However, I can understand why earmarks might be an important political issue. For one, $18 billion, on an absolute scale, is pretty big. It's akin to the federal government giving a special interest group one large corporation every year. For example, would we be upset if the federal government gave away Valero, the largest publicly traded oil refining company in the U.S., at approximately $18 billion, to the citizens city of Omaha, Nebraska? Should that be a valid exercise of government spending?
The answer is no, of course. The issue with earmarks is not that they are expensive in relationship to all other government spending, it is that they only benefit certain interest groups, instead of the country as a whole. It is not a matter of fiscal policy, it is a matter of social justice.