Much different though. With Far Cry you're talking about basic modifications on an existing template (elevation changes + pre-made objects). In UT3 you can do absolutely anything. You could build a scale model of the statue of liberty to put in your map, and you could put it in your map without a problem. Many maps (Krodan, for example) are based on meshes that are not taken from UT, but created by the user seperately. This makes the files dramatically larger than what you would find in a map created in far cry. The most important part is that the quality of connection for the people in the server doesn't suffer. When you're downloading such large files epic can't assume that people are going to have sufficient connections to allow an 8-16 player game AND having people downloading maps off the server at the same time without performance suffering for those in the game.
Ideally it would recognize the map in the game then would go to another server to download it, but if they implement such a system it's understandable that it would take some time.