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: 
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>
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