I remember (perhaps wrongly) an AirBNB update being several megabytes, implying the entire app. Today's update is only 129 KB. Are they allowed to just update portions like MacOS updates?
The iOS App Store supports delta updates since iOS 6.
In iOS 6 to 7.0.x, changed files are downloaded when updating an app. Since iOS 7.1, only the changes in each file are downloaded, reducing the update size even further.