Ah - so the next incremental will be effectively be a new full backup of the new volume ?
This way you keep a chain going (to maintain the historical restore points) but size wise there will be effectively 2 x full backup (the pre grow VBK and the post grow incremental - at least until retention time rolls through and the post grow incremental merges into the VBK)
The reason am double checking the scenario is that the drive involved is a large data volume currently ~ 9TB in size (current VBK is ~ 2.4TB)
The VEB backup is to a VBR repository across a firewall'd LAN infrastructure - so a "full" sized incremental will take some time to run
The VBR repository also has a backup copy job used to push the VEB backup across a WAN to another off-site VBR repository for DR, which will take a number of days for a full" sized incremental
(and the DR repository will need space for 2 x full backup as well)
We looking at adding another 1TB of free space to the volume
I guess the additional 1TB of free space will have little effect on backup size, it just the side effects of the above that will have impact on; time for the 'post grow incremental' transfers to complete and temporary impact on backup storage space (until that washes through)