That’s why I said that many people get confused.
When you add disks to, or swap disks out of your RAID, you have to look at the pool (the level above the volumes) , marked in your case with “Storage Pool 1”, here you add disks and/or increase the size of the pool
P.S. it looks like you are using cache, make sure you have good backups, as … cache is prone to fail (even more so when used on single disks) and will result in reduced speeds (on QTS) due to bad destaging of dirty blocks.