David Leonhardt spells it out clearly:
As a rough estimate, the government will need to find spending cuts and tax increases equal to 7 to 10 percent of G.D.P. The longer we wait, the bigger the cuts will need to be (because of the accumulating interest costs).
Seven percent of G.D.P. is about $1 trillion today. In concrete terms, Medicare’s entire budget is about $450 billion. The combined budgets of the Education, Energy, Homeland Security, Justice, Labor, State, Transportation and Veterans Affairs Departments are less than $600 billion.
This is why fixing the budget through spending cuts alone, as Congressional Republicans say they favor, would be so hard.
Here is related commentary.