expansion not happening

you have
_______
|1T|1T|___
|1T|1T|1T|___
|2T|2T|2T|2T|
-------------
 4T|3T|2T|1T

Traditional Raid5 will look like this:
_______                                        _______
|i |i |___                                     |i |i |___
|i |i |i |___  => take away parity of RAID5 => |i |i |i |___
|2T|2T|2T|2T|                                  |2T|2T|2T|P |
-------------                                  -------------

So |2T|2T|2T|2T| in a raid5 can be approximated like this |2T|2T|2T| P|
|2T|2T|2T| P| which is 2+2+2=6T, which is about 5.4 TiB

FlexRAID and XRAID Raid5 will look like this:

_______                                        _______
|1T|1T|___                                     |1T|M |___
|1T|1T|1T|___  => take away parity of RAID5 => |1T|1T |P |___
|2T|2T|2T|2T|                                  |2T|2T|2T|P |
-------------                                  -------------

So this has |1T| and |1T|1T| and |2T|2T|2T|
So this should give you 1+1+1+2+2+2 = 9T which is about ~8.1 TiB <-- this what you should have

Even if that top layer doesnt get used (because who wants raid1,raid5,raid5 its more symetrical to have raid5,raid5 and wait for another device so that you get raid5,raid5,raid5) you will get 1+1+2+2+2=8T which will be ~7.2 T

NOTE: raid5 works with 3+ devices, however raid1 can be used which can quickly convert to raid5 if more devices are added

LEGEND: i ignored, T TB (1000 based), P ignored due to parity or mirroring
T is 1000 based (Terabytes) - drive manufacturers list these
TiB is 1024 based (Tebibytes) - operating systems and the NAS use this

SUMMARY: you should have 8.1 TiB. If your getting 5.4 TiB looks like expansion didnt finish. Can you provide your logs?