Google Enables 'Smart App' Updates in Play Store
Google recently turned on a feature of the Google Play Store that will reduce the amount of data downloaded over wireless networks when apps need to be updated. The Smart App Update feature, first announced at Google I/O in June, means that applications requiring updates will only download the portion of the application that is different. For example, if only 40% of the code of a 10MB app is updated, Google Play Store will only download the 4MB that is different rather than the full 10MB. The change will reduce strain on cellular networks and Google's own app servers.
from Google Play
more info at Android Police »