"Memory usage limit reached" while unpacking libpng

On one of my older boxes.. actually a old P2 used as a firewall, I was upgrading libpng to libpng-1.2.43-r2. I received an interesting error:

>>> Unpacking source...
>>> Unpacking libpng-1.2.43.tar.xz to /var/tmp/portage/media-libs/libpng-1.2.43-r2/work
xz: /var/tmp/portage/media-libs/libpng-1.2.43-r2/distdir/libpng-1.2.43.tar.xz: Memory usage limit reached
xz: Limit was 62 MiB, but 65 MiB would have been needed
tar: This does not look like a tar archive
tar: Error exit delayed from previous errors
* ERROR: media-libs/libpng-1.2.43-r2 failed:
*   failure unpacking libpng-1.2.43.tar.xz
* Call stack:
*               ebuild.sh, line   54:  Called src_unpack
*             environment, line 1219:  Called _eapi0_src_unpack
*               ebuild.sh, line  605:  Called unpack 'libpng-1.2.43.tar.xz'
*               ebuild.sh, line  444:  Called _unpack_tar 'xz'
*               ebuild.sh, line  366:  Called assert 'failure unpacking libpng-1.2.43.tar.xz'
*   isolated-functions.sh, line   14:  Called die
* The specific snippet of code:
*              [[ $x -eq 0 ]] || die "$@"

I've never seen this before? So xz is reaching it's limit on memory? I see that there is a option to raise this level.. and the level is set at percentage of available RAM:

The absolute value of the active memory usage limit can be seen near the bottom of the output of --long-help. The default limit can be overriden with --memory=limit.

So, I looked in the ebuild for a spot where I could set this.. no dice, but I found that it is a available option when emerging (or add it to make.conf)

I executed the following command, and I was good to go:

XZ_OPT="--memory=max" emerge -1 libpng