Re: how to reduce unity 2d game's apk size
You can try to compress your audio or to convert it into wav (that is always works for me). Also you optimize your sprites and textures (for example, you can load then into Photoshop and use "Save for Web" option).
I think you should not put your attention on the code because you will not decrease game's size significantly

Don't forget to post here a solution that you approached

it will be useful for others if they meet such problem.