Sites that use more bandwidth are

1. Defintely high traffic / usage sites.
2. Very rarely clients might have uploaded huge files or other files such as vidoes, mp3, zip etc.

It is good to take this up with the client and investigate further.

If the website is high traffic / usage one, then it is better to upgrade to a corporate or a semi dedicated account. The reason for high traffic / usage is due to number of hits to the website is high or usage in ftp, email is more. Such high usage will spike the cpu and memory usage. This will affect other websites that are hosted in the server. In a reseller hosting, all the websites are alloted with equal resource and any site that spikes the resource beyond a limit for higher amount of time is considered to be monopolizing the server resource. In such instance, these accounts have to be upgrade to a higher account as it will affect other websites hosted in the server. Moreover such websites cannot function properly in a restrictive environment such as reseller hosting. It is better to upgrade such websites to a higher account as it will increase the performance of the website. The client will also be satisfied and stick with you. If continued to be hosted the same environment, the client might eventuall move out.  Upgrade options are available for resellers with huge discounts. That way you will also make handsome profit out of upgrade.

If the usage is due to mail server access, it is better to move the account to a corporate mail server account where in higher usage of such resource is allowed. 

If the high bandwidth usage is due to vidoe, mp3 or other rich media files then it is better to move them to a streaming or web2.0 hosting accounts. These formats are not allowed in shared hosting as it spikes the server load severly.

In case of any clarification required for choosing the correct package for upgrade, please contact support by creating a ticket.