Remember that the aid is actually primarily towards those still alive and affected by the tradegy not to the dead. So that's 2-3 million people who are homeless.
As for why the US has now increased it's intial pledge to levels similar to other countries and is so active in making sure that it does not appear in any way to be stingy (and i would not suggest it was), I believe that Powell said it best,
Muslims had "an opportunity to see American generosity, American values in action", and US relief work should also "dry up pools of dissatisfaction which might give rise to terrorist activity".
It's in America's best interests both to be generous and to be seen as being generous.
Paul.