Inconsistent app update sizes in App Store and Launchpad
The App Store supports compression and differential downloads that saves you time and bandwidth by only downloading the differences between the current version and the version, you already have installed. 

This explains why the updated version can be listed at 7.8 GB while only 3.59 GB are downloaded.

Note that it is not possible to download differentials in all cases. In those case you download the full package. Also note that if a differential download fails (for example due to the internet connection breaking up) - then a full download is attempted on the next try.