Until recently the Cisco ASA had a Top Talkers facility which shows you who is using up your bandwidth.
Now that's no longer available, one quick(ish) and easy(ish) way is to:
Connect to the ASA with Putty and capture the output of "show conn" to a CSV file.
Then edit the file with a text editor (I suggest Notepad++) and replace " bytes " with ""
Save the file, then open it up in Excel and you can sort by size and work out who's using what...