Hello,
In order to analyze and find out which user is using the most bandwidth on you server, i can install and configure the AWstats software on your FTP server.
Here is the webpage of AWstats: [login to view URL]
and here is a DEMO page for FTP usage:
[login to view URL]
As you can see from the demo page, you can spot which users have authenticated and which users downloaded the most files. You can query by hour/day/week/month etc. Another option would be webalizer log analyzer. I can work with you to implement both and you can decide which one to go with although i would suggest AWStats.
Kindly let me know if you have any questions, i am available today to install the software.
Thank you!
Emre.