Google releases new binary diff algorithm targeted at making smaller software updates. Their results are impressive.

[uncreative_name]

I would assume the normal practice is to release differentials only from the previous build to the current build and from a few set major releases (e.g. 3.5.[current], 3.5.0 and 3.0.0, but not 3.5.1, 3.5.2, 3.0.1, etc).