Well only developers can answer u this, i dont really think that they have free time to answer your question...
Flash games are big, but mine suggestion is when u load it once u dont close a window until u end ur daily job at farmville. Ur are downloading some parts of software ( if not all ) everytime in your temp folder. That downloading is making your bandwidth go wild. Dont know actually if hole .swf file downloads first on your PC ( That would kill ur bandwidth for sure. )
So i guess that ur downloading every time u log in at FarmVille, lets say 300-400KB or even more.
Ofcourse this might not be true, as i have said in first line only developers can exactly answer ur question.