Normally, when an update to an application comes out, the Play Store downloads it in full, which with large apps and slow connections can get quite painful (think 50MB+ sizes). Smart app updates, announced at an I/O session in June, was the solution. When enabled, only the incremental difference (a.k.a. the delta) between the old and new apks would be sent over the wire, thereby saving huge amounts of data for both Google and Play Store users.
For example, ezPDF Reader app weighs in at 6.34MB but the delta update is only about 2.7-3MB. Instagram weights in at around 13MB but the delta update is about 3MB.
Although Apple has introduced delta updates for Software Update, it has yet to offer the feature for apps downloaded through the App Store.