The Biggest American Embassy in the World

An item in the story announcing Negroponte's nomination as Ambassador to Iraq has peaked my curiousity. It says:

The United States now rules Iraq through the Coalition Provisional Authority but will retain enormous influence through its new embassy in Baghdad, which is expected to have about 3,000 employees and will be the biggest American embassy in the world.

Imagine that. The biggest American embassy in the world. Bigger than Mexico, China, Russia, Britain, and many other countries where we have ongoing and vital interests involving immigration, trade, foreign policy, etc. What exactly will those 3,000 people be doing and why does it take so many of them to do it?