Depending on your plan configuration, billing for disk space is based on either disk quota or disk usage.
If your disk space billing is based on disk usage, the Statistics section will display a bar showing average summary disk usage since the beginning of the billing month.
In this section you will also see the summary disk usage for the last day. It is the total disk space used by files in your FTP home directory, all your mailboxes and databases at the moment of the system scan. As a rule, the scans run every night.
If your disk space billing is based on disk quota, the Statistics section will display a bar showing the amount of disk space actually used out of the disk space allowed by your account quota. This bar shows disk space only on the web server.
To change your disk quota:
Read more about disk space.