Question: I made a game with Unity but the size of .apk file got so large, it is about 70mb...
Is there some way that I can compress my apk file?

Answer: 
1. You can use [Tinypng](https://tinypng.com/) to decrease you graphics size without visible graphics quality damage, Decreasing graphics size ultimatly leads to decrease in apk size.
2. Make spritessheets either using [Texture Packer](https://www.codeandweb.com/texturepacker)
or using Unity3d own [Sprites Packer](https://docs.unity3d.com/Manual/SpritePacker.html).
3. (Optional ) if you are targeting only ARMv7 or x86 Devices then change it from player Settings it will reduce approximately 8 to 10 MB.

> 
> Edit > Project Settings > Player 
> 
> 
> In Other Settings Section You will find Device Filter
> 
> 
>
Set your image size square and
Set unity suggested pixels.(32, 64, 128, 256, 512, 1024, 2048, 4096, 8192)
If your image size is 567x890 pixels then set this image in a 1024x1024 pixels background transparent blank image and save it as a png file.

Then in unity inspector set this image as a sprite and slice it by sprite editor. This process will make your image size reduce minimum 20%.

Example image and changes given below.

Before: resolution 567x890 pixel, size 1.9MB

After: resolution 1024x1024 pixel, size 1.3MB

Result: size 0.6MB reduced, almost 32%.

Image file Link: <https://drive.google.com/file/d/0B9zkzr6JdNYlaHhqcDVUSk9KNjA/view>