I think it's a waste of space that we download a 163MB tarball, that become 823M when extracted.
Specially if you use tmpfs for /var/tmp/portage
I inspected the files and 612M is for third_party.
Since gentoo most often use system libs, why not to wipe out libraries that won't be used and repackage the source?
We still need most things from third_party. Especially WebKit and icu. If you're not happy with the current state of things (which I can easily understand), please contact upstream. However, I think that bugs are filed to unbundle most of the remaining libraries.