Google said back at I/O that the play store will be getting the option of Delta updates soon, wherein only the changed part of the app would be downloaded instead of the whole app in case of an update. With the new updated Play Store app every time you decide to update an app only the changed portion will be downloaded to save time and bandwidth. This is a great feature, just consider updating a graphically intensive game when there’s no hotspot in range. There’s a video after the break, showing the delta updates in action.
Smart app updates is a new feature of Google Play that introduces a better way of delivering app updates to devices. When developers publish an update, Google Play now delivers only the bits that have changed to devices, rather than the entire APK. This makes the updates much lighter-weight in most cases, so they are faster to download, save the device’s battery, and conserve bandwidth usage on users’ mobile data plan. On average, a smart app update is about 1/3 the size of a full APK update.
Source: Android Police