About a month ago we updated our tools to support packages that are compressed with the xz format, also known as LZMA2. Compared to gzip this will decrease package size significantly; about 30% smaller or even more depending on the data.
If you want to compress your packages with the new format change the PKGEXT variable in the makepkg.conf of your build environment to '.pkg.tar.xz'. Our tools like dbscripts, devtools, namcap and of course pacman are able to process these packages. If you use any custom scripts which match *.pkg.tar.gz files, make sure to change the pattern to *.pkg.tar.*.
To allow a smooth update for older installations, for the time being the packages pacman, pacman-mirrorlist, libarchive, libfetch and xz-utils are kept with gz compression. (This is achieved by adding "PKGEXT='.pkg.tar.gz'" to their PKGBUILDs.)
At this time, all Developers and Trusted Users are strongly encouraged to switch to xz format.