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: 
You can compress certain assets individually.

After building the project, go into the log file and there will be a summary of what assets were packaged with the apk, and their sizes.
(On windows, located in C:/Users/username/AppData/Local/Unity/Editor/Editor.log)

Find large textures/music/sound, and select them in the project view to bring them in the inspector.

For each texture, there are multiple options:
Reduce the 'max size'/'max resolution' (default max resolution is 1024^2, reducing this will force unity to resize the texture upon building the project)
Change the compression (Avoid setting textures to TrueColor except where needed)

For music/sound, make sure the format is "Compressed", and then you can adjust the compression value. Compression of ~80kbps is normally good enough to cut the size down, and still mantain decent quality. You can go lower if you really want to. If the 'force to mono' checkbox is enabled, check it. It will reduce the footprint of the file by combining the sounds from the left and right channels.

Be sure to test the game with the assets compressed, and make sure things still look 'good'. You will lose quality by using lossy compression such as reducing resolution or using highly compressed texture formats.
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>