[linux-lvm] Why can't I extend a LV after many times extends?

[lepton]

There are two ways to do things:

one is:
 
  lvcreate -n test testvg -l100000   /dev/sda1:0-31 /dev/sdb1:0-31
  /dev/sdc1:0-31 /dev/sda1:32-63 /dev/sdb1:32-63 /dev/sdc1:32-63 ....

another is:
  
  lvcreate -n test testvg -l32 /dev/sda1
  lvextend /dev/testvg/test -l+32 /dev/sdb1
  lvextend .....

I found the second way can't work when the size of lv is big enough
(about 26G) it complains about metadata size is too big.

But I thinks the disk usage should be the same. why is metadata size
diffrent?