Why are updates so extremely large?

I'm currently downloading the update for iMovie (2 GB) and Xcode (3.5 GB). Other similiar producs are maybe several 100 MBs, but for the full software, not just an update.

• What makes those updates so extremely large?
• Do they typically replace existing files or do they actually consume additional 2 or 3 GB on my harddisk?

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.