zack@aquarium:~$ dpkg -s python-django|grep Installed-Size
Installed-Size: 7776
zack@aquarium:~$ du -sh /usr/share/python-support/python-django/
7,2M    /usr/share/python-support/python-django/
zack@aquarium:~$ du -shc /var/lib/python-support/python2.*/django/
4,8M    /var/lib/python-support/python2.4/django/
5,0M    /var/lib/python-support/python2.5/django/
9,7M    total

Disclaimer: I'm fine with python-support and friends, they've smart architectures. However it's a fact that currently a huge deal of packages in the archive are size-wise not faithfully represented by the Installed-Size field (and the ratio can be quite impressive!). I wasn't here when that field was designed, but I bet an intended use case was to give admins a way of knowing which packages were wasting disk space the most. For sure dpigs relies on Installed-Size and don't have the necessary info to identify stuff like the above.