This is the way updates and patches work everywhere else: You just 'patch' the parts of the executable(s) that changed, not replace the entire file. Okay, most of the time, as there are still a few dinosaurs who prefer to send you the entire executable to replace instead.
Now Google Play works with that new paradigm too, which is a win-win for everyone: less web traffic so Google and the consumer can save money, and faster installations meaning happier users.
Reshared post from +Patrick Sharpe
Google Plays Sneaky Update:
I was updating something from Google Play yesterday and it was around about 25 megs. Normally on my amazingly slow connection it takes about 15 mins for a file of that size to download, but not this time. Before it finished downloading, or going the full length of the download bar, it had already started installing itself. What? I thought something must have gone wrong so I loaded up the app to make sure. The app ran fine and so I just chalked it up as an oddity.
Apparently Google updated Google Play without requiring its Android user base to download an update for it. You know what that means kiddies? It was a server side change. Google Play now allows for incremental updates. Rather than having to download the entire APK file for each app, which is what we were all doing before, now we'll only be downloading the incremental changes significantly reducing the size of our updates.
It's great news for those of us with slow internet connections but even better news for people who use their data plans to update their mobile devices. Sweet!