Google do not want your users to have to spend a large amount of data in your rate to upgrade your applications and games when they can not update for Wi-Fi, and for this Optimizing the size of download the updates again.
Last summer managed to reduce the size of the updates in an average of 47% thanks to the algorithm bsdiff (by Colin Percival) which has now been enhanced with the new patched file by file, thus reduce the size of the updates in an average of 65%, and in some cases by more than 90%.
The new update method only download the parts that have changed in each one of the files containing the APK file, getting the update patch to occupy less than half, but now installation time will be doubled.
Installation time will increase because in order to apply the update patch you will need to first extract the APK from the device, apply the patch and then compress again the APK. This process in most modern devices can take a second MB of the size of the application, but more in the older devices.
To avoid affecting the performance of the devices, These new patches will only apply to automatic updates. As soon as the device is connected to the current, and usually early in the morning, it will automatically upgrade applications with this new system, provided you have enabled automatic updates.