Is it 620MB of source text? I'm not a web guy, so I literally I can't understand this, unless, since it's a web tech, 30 minutes of cat videos are required in the source.
Yes and no; there are some binary files too, but there are ~534MB of JavaScript code mixed with additional assets. Once you extract the content from the archive (using the tar.gz version) you will find a directory with multiple sub-directories and files that roughtly weighs 636MB distributed like this:
The directory "resources" contains the ASAR file plus another sub-directory named "app" that contains the assets and the "node_modules" tree, this directory alone weighs 534MB because of the 720 NPM packages required by the application.