After the first full backup, file transfers appear to be similar to those required for an incremental backup. More directory information is transfered. Incremental backups only pick up new and modified files since the last full backup. You need period full backups to track file and directory deletions. The transfer overheads are similar after the first full backup.
I just setup backuppc myself. Rsync has the nice feature that if both systems already have a file, it doesn't need to be transferred. There is a long period at the start of the backup where the information to determine which files need to be transferred is determined. The first full backup will transfer the whole file system. From my review of the documentation 1% of the files are transferred and compared to ensure pool is OK.
Rsync uses modification time to determine if the file has changed. It may use other criteria as well. Backuppc uses the same criteria.
Full backups build a full directory tree so file deletions are fully tracked. After the first full backup the transfer volumes are significantly lower. The backup speed seems similar to that of an Incremental backup.
Incremental backups don't record the information required to determine if a file has been deleted. They appear to have a relatively sparse directory tree sufficient to hold the new and modified files.
EDIT: I triggered a full backup on a system with only an incremental backup of most files. It ran very quickly and linked most files as same, rather than copying them. This is what I expected. Network load was significantly less than for the initial transfer.