When does the pool get bigger?

[Zaphod Beeblebrox]

I have a ZFS raid-Z array (FreeBSD-7.1p2) that I use for storing backups and
media.  I'm keenly awaiting the MFC of the ZFS v13 code, but I'm not in a
hurry to run -CURRENT on this box.

Anyways... The array was 5x 750G drives and I decided to upgrade to 5x 1.5T
drives.  I removed one 750G drive and inserted a 1.5T drive each time.  All
5 are done resilvering now.

When does the pool get bigger?  The resilver of the last drive has finished,
but the pool still reads

[1:20:320]root at virtual:/usr/local/etc> zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
vr2                    3.41T   3.16T    251G    92%  ONLINE     -

... which is the size with 750G drives.