Because it bundles Electron [1] which weighs (for version 1.2.6) around ~40MB, if you also include the size of the 720 NPM packages [2] that this application uses then you get the same size that you are complaining about. However, if you also extract the content of the archive you will notice that it actually weighs ~620MB LOL.
[1] https://github.com/electron/electron/releases/tag/v1.2.6
[2] http://pastebin.com/raw/VhnuuDdX