Most of the app bundle on iOS are various assets (.pngs, videos, sounds, etc). This looks like it's drawing everything in code, so the app bundle is really small.

Marco's new app, Overcast also has UI drawn almost entirely with code (all icons are generated with PaintCode [1]), and his app [2] is 4.5mb.

[1]: http://www.paintcodeapp.com [2]: https://itunes.apple.com/pl/app/overcast-podcast-player/id88...