Depending on your computers Windows UAC security settings you may not be seeing your entire file selection.
First make sure you are logged into your Windows user account that has access to the data you want backed up or make sure that you are logged in as an admin.
Also there is a link that you will find above the "Help, Ok, and Cancel" buttons in the Mozy settings called "Change settings that are currently unavailable" colored blue. When you click on this link in the Mozy settings, it will close the Mozy settings and reopen the settings allowing you through Windows UAC security so you can see all files selected for backup.
Once the Mozy settings reopen you can see the total file selection size reporting next to where it says "This Device" at the bottom of the Mozy settings in the "File System" and "Backup Sets" tabs.