Google Play now displays the actual storage space a whole app or an upgrade will take up, so you don’t accidentally download anything too big. If an update is only 2.91MB, it will show that exact figure right there in each app’s detail box. That will give you the chance to reconsider your download or to free up some space before getting a particularly large game or VR experience.
Besides displaying more accurate file sizes, Google also tweaked its Play Store algorithm to make updates even smaller. Most Android apps (98 percent of them) only download changes to their APK files when you update them, and those new files merge with the old ones. The updated algorithm will make those updates up to 50 percent smaller.
Finally, Google’s improved compression algorithms will reduce big games’ file sizes, which could be as huge as 2GB, by around 12 percent. Those with high-end phones might not get much out of these changes, but they could make a big difference for those who own more affordable devices with limited storage.
Source: Android Developers Blog