Haha, perfectly valid question that was never asked by anyone in 5 years
Backup mode has nothing to deal with this, it is how backup files are initialized. Veeam backup file has pretty complex structure - it's not just raw data file, but single instance block storage with support for transactional updates and some level of redundancy. When the new backup file is initialized, it pre-allocates 8 MB and puts the required data structures in place.
Unless you are backing up turned off VM, you will probably never have the size of data in backup smaller than 8 MB anyway, because quiescing the operating system and apps will make more than 8MB of blocks "dirty".