Why are updates so extremely large?
Almost all apps downloaded from the Apps Store are full (self-containing) updates or upgrades. They completely replace the updated app.

The only exception updating your system with the App Store.app are security updates (e.g Security Update 2015-002) or system updates (e.g. System Update 10.11.1 or Combo Update 10.10.5) which don't necessarily replace an app/framework/resource but often only parts of it.