It's certainly, something, alright. Feels like a circular pinball machine that will give the player a headache haha. But there is no real gameplay yet? Pressing space bar doesn't do anything.
Anyway, to get back to the main topic, with most unity games, the initial size of the game is mostly the build-in engine to play the game, meaning most of the 15mb is just the unity player. And from my experience, the thing that increases the filesize the most in amateur games are audio and image textures.