Yes, that is true, in dollar amounts, since you have the largest population.
If you go by percent of GDP or by capita, the US does not come close to donating the most.
here, from 2005, (as you can see in billions, the US is on top, GB second).
Anyways, you are forking out less foreign aid as a tax payer belonging to the US, then those of GB, Sweden, and even Canada.
For example, if everyone in a small town of 1000 people donated $1000 each = $1 Mill.
Now if the same charity event took place in a big city with 1.2 Million people and each donated $1 dollar, (= 1.2 Mill), you cannot say that the average townsfolk donated less !