I am not worried about the deficit...
I am not worried about the deficit. It is big enough to take care of itself.
This joke refers to the national deficit, which in the U.S. is known to be very large. The joke states that the jokester is not worried about the large deficit because the deficit is large enough to take care of itself.
In English, "he can take care of himself" is a colloquial phrase that means "he is old or mature enough that he does not need supervision any longer." This seems to imply that the deficit is a person that was old enough to no longer need supervision.