now dpkg-deb uses xz for compression by default

from dpkg 1.17.0 's changelog

 dpkg (1.17.0) unstable; urgency=low
 .
   [ Guillem Jover ]
 (snip)
   * Switch dpkg-deb default compressor from gzip to xz. Build dpkg.deb using
     gzip to make debootstrap life easier on non-Debian based systems.

Hallelujah, thanks! It'll reduce archive size and cut traffic, download time, too :-)

Then, I hope that dpkg would support "package delta" feature like yum's presto.