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
> 
> 
>
You can also choose to set the Device filter to Arm7. This will exclude certain devices, but I believe the number is quite small and it does minimize the apk size a bit. (This is located in Player Setting -> Other Settings -> Device Filter). You could try a build and see how much it would reduce it by and decide whether it was worth it.

Some Info on device filter: <http://answers.unity3d.com/questions/971648/device-filter-to-arm-7-only.html>