Proguard doesn't reduce apk size

I'm in the process of making my app ready to release. Currently I'm fighting to get Proguard running.

I enabled Proguard in the project properties of my Android startup project, updated to Proguard 5.3.1 and added a symlink so that it works in a folder with a space in the path.

When I build my project I get these warnings:

PROGUARD : warning : can't write resource [META-INF/MANIFEST.MF] (Duplicate zip entry [formsviewgroup.jar:META-INF/MANIFEST.MF])
PROGUARD : warning : can't write resource [.readme] (Duplicate zip entry [classes.jar:.readme])
PROGUARD : warning : can't write resource [.readme] (Duplicate zip entry [classes.jar:.readme])

There are no errors, but the resulting .apk (after jarsigner and zipalign) has the same 30MB which it already had without Proguard.

Did it actually do anything when it wrote those warnings? I didn't see any errors in the diagnostic build log.

I use VS 2015 professional, so I don't have the options to AOT/LLVM. One package per ABI is not enabled yet. The linker is set to SDK only.