You'll see that manditory programs (those defined by law) account for 56% of the budget in 2010 - so it's not like Obama can wave a magic wand and make the costs of government go away. They have to be voted on by Congress.
And if you look at the percent of budget going towards Medicare, Medicaid, and Social Security as part of manditory programs, it ends up being nearly 70%. So if you really want to shrink the government, get rid of the backbone of our country that supports the elderly. Try suggesting that and see how far you get. Social Security and Medicare are pretty popular programs.
You'll also find that security is 23% of the budget. We can shrink that by closing the wars - oh, right, Obama is doing that....
If you look at everything else the government spends money on - that isn't related to security, and isn't related to manditory spending - you're left with 15%.
So, obviously, you can't shrink the government down to 1/2 or 1/3, even if you gutted every single dollar of every single program that the government can legally do without an act of Congress. You'd only reduce by 15%, and some of those programs - like roads, disaster relief, and unemployment - are things you might want to consider keeping.