See MaxedOutMama's analysis of the Obama budget:
The numbers behind the, ah, "budget" can be read here. Start with the tables (22 page pdf file). Go to page 5 and look at that chart. The first section is the baseline projection of outlays. The huge budget item (a fact that Democrats don't want to see) is the social welfare spending. That includes Social Security, Medicare, Medicaid, and all other welfare programs such as food stamps, other medical grants, and welfare.(via Chas' Compilation)
In 2008, the outlays for these programs ALONE are 2.9 trillion. Yes, that is TRILLION. In 2018 the projected outlays for just these programs are just under 5 trillion. Current dollar US GDP as of Q4 2008 was estimated as 14.26 trillion. Thus, we are spending slightly over 20% of GDP on just the entitlement programs. That doesn't include defense, and it doesn't include interest on the deficit.
In 2008, interest on the deficit is listed as 253 billion. In 2018, interest on the deficit is expected to cost 651 billion.
In 2008, the deficit was 459 billion. Now we go to table 1 in the pdf, which shows the proposed spending. The proposed 2009 deficit is 1.7 trillion. In 2010 the proposed deficit is 1.1 trillion.