NAS326: Upgraded my Single 2TB HD to 4TB RAID1, but volume size hasn't changed.

Hello All,
Can someone help offer some adivce with this please:

My NAS326 used to contain a single 2TB drive (WD Red).
Yesterday I decided to upgrade to a dual 4TB RAID1 setup, so I bought 2x4TB WD Reds.

I put one drive in bay 2, and created a RAID1 disk group. The mirror was built no problem at 2TB
I turned off the NAS, removed the 2TB drive and replaced it with the other 4TB drive. Booted up the NAS, used the repair RAID opion and let the array rebuild over 8 hours.

Everything showes up fine, except my volume is still showing only 2TB (1.79tb) instead of the 4TB (3.64TB) I was expecting.

I selected the volume and pressed EDIT.  I was given the option to increase to the max size for group 1 and selected that.  Now I can see Disk Group 1 Usage is 100% 3.63TB, but directly below shows that the status of the volume is still a total size of 1.79TB.  I can no longer use the edit button.

I'm stumped here.  I'm not sure how to use the full volume size.

Can anyone tell me what I'm doing wrong to allow the full usage of these new drive s please.

Thank you

Here are some sceen-grabs

#NAS_Jan_2020