Using webmin, you can get awstats or webalizer to work easily...
Then, i.e. on awstats, you can see on your top urls list, which downloads are the most "downloaded". Also, on the file types list, you can see which kind of file type is the most downloaded (images, isos, pdfs), you can tune the config for getting more detail, and avoid that awstats exclude images or some kind of file type from the stats...
Also, easier on the standard install, webalizer generates a "Top ten urls by kbytes", that shows you which exact file is using the most bandwidth...
Edit: I saw your comment on Antoine answer, so I'm going to mark bold the most interesting approach for you, I think...