To understand how it is calculated, consider account Tony is assigned 50 MB disk quota. When this user does FTP to his account ¬Tony¬, the operating system check if Disk Quota is assigned to this account. It finds 50 MB disk quota for this account, therefore when the user uploads data to his website, operating system calculates the total amount of data uploaded and stops the user from uploading more than the assigned 50 MB disk quota.
As described above, the disk quota feature is based on FTP only therefore if user creates MS SQL database, the database can grow to any size unless restricted using MS SQL Enterprise Manager. Similarly there is no restriction if user¬s log files or mails grow to even GBs.