Google Play app updates now accurately reflect download sizes6
Instead of re-downloading an entire app each time a new version comes out, four years ago Google gave the Play Store the ability to deliver “delta” updates to our phones and tablets, containing only the parts that had changed between the old app we had currently installed, and this new version. If a 50MB app added some new icons, instead of having to download the complete 50MB package when updating, Google Play would instead update just those icons – a much smaller download.
Now that's finally changing in order to only show the size of the delta update – the actual amount of data your phone will download when updating. Especially if you're near the top of your monthly data cap, that could be very useful information to have, letting you know if you can grab an app's latest update without exceeding your plan's limits.
If the version of an app on your phone is already up to date, Google Play won't display any file size for it at all. But if you do need to install an update, the file size you see should now accurately reflect what you're about to download.
via: Android Police