File size increases on different devices

So we’ve made a rather simple project for mobile and want to keep the file size as small as possible.
We’ve followed a lot of optimization processes and got it to arround 20 MB.

Unity editor log says it’s 20MB
the .apk says 20MB
when building on a tablet with android 4 it says 20MB

but when building on phones with android 5 it is at 58MB and we can’t do anything to get it lower.
It’s already strange that there is such a big difference.

Any idea what could cause this problem?

Immediately after making a build, load the editor log (from icon at top-right of console) and scroll near the bottom - it will give you a breakdown of what’s using up all the storage in a build.