When Google announced Android 4.1 Jelly Bean , it also introduced a new feature called Smart App updates for the applications downloaded from Play Store .Now, the Smart App updates feature has been activated for App updates .
Previously , when getting an updated version of App , Play Store used to download full new package which was as big as the App itself and this wasted time and bandwidth . With Smart App Updates , the downloads are only about one third in size containing incremental data , which means much lesser time required to update apps .
The Smart App Updates is being used for some apps already on the Play Store .
Source

Previously , when getting an updated version of App , Play Store used to download full new package which was as big as the App itself and this wasted time and bandwidth . With Smart App Updates , the downloads are only about one third in size containing incremental data , which means much lesser time required to update apps .
The Smart App Updates is being used for some apps already on the Play Store .
Source