I noticed the downloading of app updates is really fast on my iPhone X (iOS 13). Like for example the facebook app shows about 250MB in size on the update screen of the appstore but it's finished downloading in about 10 sec over WLAN in flight mode. Despite I have 40Mbit/s (5MByte/s) internet speed. So it should take about 50 seconds for this download. I am sure the update file(s) is/are compressed but this would mean like 250MB/5 = 50MB, which sounds like too much of possible compression for an app file. I mean it's not a text file with many redundancies.
So I wonder if updates contain only changes to the app but then the programming logic would have to allow that which I doubt that many developers do this because I saw this behavior not only in the facebook app but in most app updates.
A more plausible reason would be that the size shown next to the update on the appstore is the uncompressed/installed size of the app. Does anyone know?