パーティションサイズ+ lvmを減らす方法

/ varには、Linux Red Hatサーバーに101Gがあります。

grep var /etc/fstab
/dev/mapper/vg10-00_var /var                    xfs     defaults        0 0

30Gに減らしたいです。

 df -h /var
Filesystem               Size  Used Avail Use% Mounted on
/dev/mapper/vg10-oo_var  101G  3.6G   98G   4% /var

だから私たちはそうします：

 lvreduce -L -70G /dev/mapper/vg10-00_var
 WARNING: Reducing active and open logical volume to 30.87 GiB
 THIS MAY DESTROY YOUR DATA (filesystem etc.)
 Do you really want to reduce lv_var? [y/n]: y
 Size of logical volume vg10/oo_var changed from 100.87 GiB (25822 extents) 
 to 30.87 GiB (7902 extents).
 Logical volume lv_var successfully resized.

しかし、xfs_growは、データサイズ8091648が小さすぎると文句を言います。

xfs_growfs /dev/mapper/vg10-00_var
meta-data=/dev/mapper/vg10-00_var isize=256    agcount=26, agsize=1039872 blks
     =                       sectsz=512   attr=2, projid32bit=1
     =                       crc=0        finobt=0
data     =                       bsize=4096   blocks=26441728, imaxpct=25
      =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal               bsize=4096   blocks=2560, version=2
      =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data size 8091648 too small, old size is 26441728

なぜ？