The two APKs are generated with:
- Android Studio 3.0 (Gradle Plugin 3.0.1, Gradle 4.1) and
- Android Studio 3.1 (Gradle Plugin 3.1.0, Gradle 4.4)
Apparently, size difference in the resources comes from binary XMLs now including the namespace attributes and "xmlns:*" attributes, that were previously stripped.
See the attached image for the differences for activity_main.xml.