As I have observed of late, the latest version of Google Play Store has been showing only the incremental sizes of the updates.
To explain this, let me consider you are installing an app for the first time, WhatsApp in your case - you will need to download the entire app. It will be in say, 20 to 40 MB depending on the app you install. Further, when your app receives an update, only a few patches are changed - not the entire code. That way, you will need to download only a few MBs to get an update.
These are called Delta App Updates. Google Play Store has introduced support for Delta app updates long ago. That meant, when a particular app receives an update - you will not need to download the entire app. You might have noticed before that though the app size was mentioned much more for some apps, the update gets completed in just a couple of seconds. That is because the size shown had been the full app size - not of the update patch.
Though Play Store had been providing Delta App Updates, the process was not transparent. The newer version(s) of Play Store has been showing the actual update size than the full app size. That is exactly the reason why you see the reduced update size.
If you are downloading an app for the first time, you will see the full app size. Later on whenever the app receives an update, Play Store shows the Size of the update patch alone. That is the exact amount of data you will download to get the update. Google seems to have introduced this feature in a phased manner. Not all devices have this feature as yet.
The point would be clear when you use MoboGenie for downloading or updating the apps. MoboGenie will show you that the app has an update and prompt you to update the same, but if you select Update, it will download the full app rather than the updated patch alone.
Please note that Play Store does not indicate the app size now for the apps you have already downloaded. It will show the sizes only for the apps which have updates available or for those which have not been downloaded. This should explain the things in a clear manner.
Google Play Store had started offering updates or the Delta App Updates long ago in 2012 when it launched Smart Updates feature. The feature has been transparent now - that is the diffeerence.
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