[Kemalony]

how to reduce size it says dll files take too much place

[Rukbat]

Write tighter code. You can't just make a file smaller - it's as large as the code in it.

[Jeremy Burk]

Changing your code is likely not going to reduce your final apk size by any worthwhile amount unless it's by removing any references to dll's that are getting included which otherwise would not be.

Your textures are probably where your size is coming from. Look into texture compression.
You can find more info here...
Unity - Manual: Reducing the File Size of the Build

[peamur]

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.

[siddigTheCat]

Compress your artwork and sound assets if you have not already done so. This will have a big impact on your apk size.

[ahushiling]

Make separate version : One apk for ARM, another apk for x86. GooglePlay support multi apk. You can save more than 3M on this small trick.