Some of your unallocated space is probably reserved for expansion - check the Droboulator for your model and put your drive sizes in to verify.
Drobo will present as many n TB volumes as is necessary to completely “cover” your total protected physical storage.
Volumes are never removed, even if your total protected physical storage reduces.
For example, say you have 2 x 2 TB drives and 2 x 1 TB drives in Single-Disk Redundancy, resulting in 3.6 TB of total protected storage…
2TB volumes: 2
4TB volumes: 1
8TB volumes: 1
16TB volumes: 1
Say now you upgrade (one-at-a-time, of course) drives to 4 x 2 TB drives, resulting in 5.5 TB of total protected storage…
2 TB volumes: 3
4 TB volumes: 2
8 TB volumes: 1
16 TB volumes: 1
The available protected storage space is shared among all those volumes dynamically, so in the 5.5 TB example, you could have two 100% full volumes and the third 75% full, or any combination of usage on the three volumes that does not exceed 5.5 TB total.
Once you set the volume size you cannot change it. In order to change the volume size, you would need to copy/move all your data off the volume, reformat the Drobo volume with the new volume size, then move your data back.
On DroboPro and DroboElite, you have smart volumes, so you could just create a new smart volume of the desired size, move your data from the smaller volume(s) onto the new volume, then delete the smaller volume(s).