This is mostly because WWWBackup keeps version history. So if you have modified a file, it will be backed as a new version of the file. Most users like this feature as it allows them to recover an older version file. However, if you don't have enough space, or if you don't to waste space, you can set WWWBackup to only keep 1 version.
First, make sure you have the latest version WWWBackup; then logon, click on Manage Tasks, select a task and click Edit, it should let you set maximum versions to keep.