ZFS swap vdev space accounting

I am on an OpenSolaris system with ZFS on it. I am trying to determine the exact space usage and/or reserved space for the swap vdev.

root@storage:~# zfs list
NAME                        USED  AVAIL  REFER  MOUNTPOINT
rpool                      74,9G  34,4G   120K  /rpool
rpool/ROOT                 8,91G  34,4G    96K  legacy
rpool/ROOT/synetostorage   8,91G  34,4G  8,75G  /
rpool/dump                 32,0G  34,4G  32,0G  -
rpool/export                448K  34,4G    96K  /export
rpool/export/home           352K  34,4G    96K  /export/home
rpool/export/home/admin     152K  34,4G   152K  /export/home/admin
rpool/export/home/zfssnap   104K  34,4G   104K  /export/home/zfssnap
rpool/swap                 34,0G  68,2G   123M  -


root@storage:~# zfs get refreservation, reservation rpool/swap
NAME        PROPERTY        VALUE      SOURCE
rpool/swap  refreservation  34,0G      local
rpool/swap  reservation     none       default

Why is swap seeing more available space than other vdevs? If I try to make swap 65G (add 33 more GB for swap), it won't let me 'cannot set property for 'rpool/swap': out of space'.