Best i can tell, what Android does is download a delta patch. Then apply that to a local copy of the previous APK. If that goes well, it attempts to install t

Best i can tell, what Android does is download a delta patch. Then apply that to a local copy of the previous APK. If that goes well, it attempts to install the new APK alongside the old install. And if that works out, the user data is switched over and the old install and APK removed. So at certain points during the install you need to have enough space for two full installs of the app, and the APKs they came in.