When removing a disk from a pool the calculation to disqualify the action is based on if the disk being removed is larger than the overall pool usage. Where as the action should only be disqualified if the disk being removed is larger than the pool free space: as the error message state:

To reproduce I added a 7.5 GB disk to a nearly empty (1GB used) 473 GB pool.
I was then unable to remove this 7.5 GB as it's size was > than the use space of the pool ( about 1GB), rather than the space free (pool size - usage).

Removing these([u'mmc-00000_0x1ceff260']) disks may shrink the pool by 7864320KB, which is greater than available free space 1081856KB. This is not supported.