I believe it's true that Americans give more of their income to charity than other countries do, but much of that stays within the USA. In terms of foreign aid, the USA is quite far down the list.
I consider this a good thing. As William Easterly of NYU and others have shown, there has been no correlation between economic growth and foreign aid for poor countries. Africa has received well over $500 billion in foreign aid over the past decades, and it is in many measures just as poor as it was in the 1970s. And I believe people have a better idea of how effectively their charity money will be spent the closer it is to home, so it is a good thing more of it stays in the US. One is more likely to be familiar with the staff, more likely to be able to see results for one's self, etc.