Sites that use more disk space are likely to be
1. Either high traffic / usage sites
2. The client might be unknowingly stacking mails without downloading it using email client such as outlook express
3. Data that is not required to run the websites in the server are stored in the server
4. The server space is being abusively used to share date, software downloads etc.
It is good to take this up with the client and investigate further.
If the usage is genuine, like the site is high traffic usage with lot of files or mails in it, then it is better to place the client under an upgraded hosting account like corporate or semi dedicated server based on the usage. Instead of trying to host such clients under your reseller or shared hosting itself, it is better to move them to a separate corporate or semi dedicated account but still keep it under your control. By doing this, the client will have an enhanced performance for his website and you can also retain the client with you. If you keep hosting the domain under restricted resource under your reseller account, the client might eventually 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 stacking mails, it is better to move the account to a corporate mail server account where in higher usage of disk resource is allowed. [ However, we strongly recommend to keep downloading mails to avoid any loss of data due to hard disk issues if any ]. Stacking of mails increases the file count in the server which in turn spikes the server processing resource due to local os indexing etc. It also affect the speed of your web-mail client access.
If the usage is not genuine and if it is abusive, like data sharing or software warez stored in the server, then it is better to remove such accounts as they will spike the server resource and invite unnecessary security threats. It is also illegal and abusive to host such websites. If the client is using the server as ftp server for data sharing, it affects the server performance drastically. It is better to move those accounts to Biz Secure FTP accounts which are specifically designed for such usage.