ACU and unused space

[Alfredo_20]

I added two 146GB disks to an existing array with a logical disk of 69.8GB (RAID 1-0 obtained from two 72GB HDs).
Now I see in ACU an unused space of 138GB, but if I try to extend the logical disk with ACU wizard I can obtain max 138GB (always in RAID 1-0).
Why cannot I obtain the size of 69+138GB as I expected from 2x72+2x146 HDs?
And if ACU doesn't permit mirroring on different sized disks without wasting extra space (refered to the smallest disk in array), how can I remove the new disks from array (they are still "unused") ad adding them to a new array?
Thanks for support and best regards
Alfredo Premi