App updates are awesome. Burning mobile data to download them isn't.
There aren't many people who turn off automatic app updates as soon as they get a new phone, but many who do say the biggest reason is data usage. If you only have 4GB of data in a month, losing 200 megabytes every month to seemingly-constant Instagram and Facebook updates is a waste of bandwidth. At the same time, not updating apps means efforts made to improve your experience have been lost until you remember to go look and see if there's an update for you the next time you're on Wi-Fi.
Google tried to address this problem not too long ago by using an algorithm that helped optimize the size of an APK. Now, that process is being taken one step further with file-by-file patching, and according to Google it's going to dramatically reduce the size of app updates.
Google has a full and detailed explanation of how this new process will work on the Android Developers blog, but the bottom line is you can expect anywhere between a 65% to 90% decrease in the size of your average app update. You'll also start to see update size information in the Play Store description, so you know what you're getting into before you click the download button.