the problem is when a custom map has custom graphics or music, music effects it more.
the map download is built more for maps using included game assets,all that those maps need are xmls telling the game where to put them.
when you get custom stuff it has to transfer music files and image files in addition, this is allot more data.
i hope that myran plans to improve the transfer system to better support custom resources. i remember him mentioning it once but at this stage i dont think its a high priority