Google talked about this feature at their I/O developer conference earlier this year, but it looks like they've turned on delta updates for apps running on Android 2.3+. These new smart updates enable users getting to sit back and not needing to download the complete app when there's an update.
Instead, just parts of the app that have been changed will be acquired, saving bandwidth, and time. When Google first announced this feature, the company's engineers estimated that the now-live feature would download around one-third the size of the app when updating. Better yet, developers won't need to touch a thing in order to have this feature enabled, as the feature went live just hours ago.
Android Police are reporting that an update for ezPDF Reader is out, which would normally come in at 6.3MB, but with smart update, only consumes 3MB. Instagram, which just hit version 3.0, would normally be a 13MB download, comes in at just 3MB post-smart update. This is a great change from Google, especially if you want to quickly grab that app and you're not connected to a Wi-Fi network.