You can use “Apk analyser” feature of Android studio. Best and easy way to start using Apk analyser is, just drag and drop your apk over android studio home screen and it will show you size being occupied by each of the things going in apk.
Another way of opening apk analyser is by
Build -> Analyze APK..
Below is the sample output of facebook apk.
If you are expecting to reduce apk size then you may refer my presentation “Generating Efficient APK by reducing size”
solved how to find each folder size of an apk in android?