Figure 1: Last three backups (2 incr + 1 full)
Support told me that the Data Size column represents the total disk size, i.e. used + free space, i.e. virtual disk itself. Backup Size represents the resulting backup file on the repository server's file system.
Figure 2: Same last three points but Report view
Support told me that Total Size in this screenshot represents the total disk size as Data Size does in the previous screenshot. And while that works for the full backup, it doesn't for the incremental.
Full backup: Data Size (1.56TB) ~= Total Size (1.6TB)
Incremental backup: Data Size (40GB) != Total Size (1.6TB)
So what does Data Size actually mean? It's obviously not the same as Total Size.
Additionally, what is the basis for Veeam's note that the compression rate was 1.5 for the full backup? Is it 1.5 of the Data Size (1.56TB) or Total Size (1.6TB) or Data Read (847.7GB)? It would seem to be the latter because 847.7GB / 1.5 = 565.13 which is approximately what Veeam is listing for the Backup Size (573.1GB). I am disheartened by this because that means the compression ratio is closer to 33% than 65%, a number on which I've based a lot of calculations across a lot of deplyments. I think it would be far more helpful for Veeam to list the actual compression percentage (i.e. 33%) rather than a factor.