EDA500 - larger drive added - incorrect expansion

I am using ReadyNasas OS 6.6.1, have an RN314 and an EDA500 attached. There are 2 volumes. One spans the RN314 and the other the EDA500.  The RN314 holds 2 x 6tb and 2 x 4Tb. The EDA500 had 4 x 4Tb drives.  Total size for EDA500 was showing is 12.29Tb.  In the eda500 I replaced 2 x 4tb with 2 x new 8Tb drives (1 at a time, resync between) and my volume size stayed at 12.29Tb. Vertical expansion SHOULD have seen a size increase. I then noticed the XRaid button was not on (implication being I am using FlexRaid). I clicked the XRaid button in the admin console. It proceed to resync then eventually hung - I could ping the NAS, but the web access failed and the button on the front did nothing (except a forced power off if I held it in for a while).

So I bit the bullet and factory reset the system, destroyed both volumes and I'm starting over (followed by a lengthy restore from backup :mansad:). Immediately after recreating the volumes I clicked XRAID so it was on and my volumes now show as 12.72Tb for the RN314 (as expected), but the EDA500 shows 14.53Tb.  My understanding is size = sum of all drives less size of largest thus 20Tb. The size I'm seeing is what I'd expect if those 8Tb were 6Tb drives!

What am I doing wrong? Should I pair the drives differently eg 2 x 6Tb + 2 x 8Tb in the 314 and 4 x 4Tb in the eda500?