considering package delta
From Android Developers Blog: Saving Data: Reducing the size of App Updates by 65%
We should consider providing delta package, especially update packages from security.debian.org, IMO.
Yes, Bálint Réczey and others via email pointed out there's debdelta.debian.net for this purpose. But for general usage, it should be intergrated to the infrastructure, without any extra manual setup. Probably apt (as Julian Andres Klode said in his talk in DebConf16and also infrastructure (dak?) would need to be modified to implement it.
Applying delta to daily unstable/testing update may be hard, but security update packages from security.debian.org and stable point release is worse for the effort at least, IMO.
Some rpm distro (Fedora and openSUSE) have already provided delta package, so we can do it, too. Right? :-)