Readynas 626x expansion

New Readynas 626x running 6.6.1.  I started with 2 2tb drives in the default xraid config.  I then added 4 4tb drives, so I now have two 2tb drives in slots 1,2, and the rest are 4tb drives.  The xraid moved from raid 1 to raid 5, and expanded the volume, as expected.  However, it appears as if it's only using 2tb on the 4tb drives, rather than expanding to use the full 4tb.  My total volume space is now 9tb, which sounds like 6 2tb drives in raid 5 config.  I'm not using encryption or anything else.  I thought the xraid would utilitze the 4tb drives to their max once I had more than 1 in there.  Do I need to replace the older 2tb drives to utilize the space?  My old Pioneer edition nas handled the incremental adding of larger drives well.

Thank you,

TJ