In IIS 6.0 default FTP Publishing Server does not log for Bytes Sent and Bytes Received because of which if we try to read W3C logs of FTP we will not able to get the Downloading and uploading bytes details.
To enable these fields do the following Steps
Right click on FTP Site
Click on Properties
Ensure that Enable Logging is checked and click on Properties button
Click Advanced tab
Select the bytes sent & bytes received check box
This will enable FTP logging along with downloading and uploading bytes details.