A Smart App Update for iOS?

We all know that Android 4.1 (Jelly Bean) will support a smart app update that only transfer modified parts of apk.
Recently I found out that maybe iOS 6 also support this.

A recent update of kindle.app, takes 10.81 MB instead of 19.9MB, and news360.app takes 5.85MB instead of 8.8 MB.
I don’t believe it is due to the compression.