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
> 
> 
>
It is also good to set texture / sprite compression format.

Check this website: <https://docs.unity3d.com/Manual/class-TextureImporterOverride.html>

Most texture compression format needs image in power of 2 resolution (128, 256, 512, 1024, 2048). Resolutions 4096 and 8192 may not work on some android devices.

For example texture with no alpha in 1024x1024 resolution using RGB ETC2 compressions takes about 0,5MB in memory. Image 1280x800 with compression format RGB 16bit takes 2MB