Create a simple demo application with instructions: http://wildfly.org/swarm/
In my case, it was a hello world vaadin application.
The created "fat" jar contained too many artifacts in the m2repo directory, e.g. all my application dependencies. This is not needed because the created war archive contains all my application dependencies?
The file size was ~ 102MB. The automatically created jar was ~ 40MB.
The overhead of ~62MB is too much!
I tried to create a "fat" swarm jar with an existing war archive (without maven build). The jar file was ~ 83 MB (the existing war was ~ 60MB). The overhead was ~ 23MB which was ok for the application server.
I had some problems with manual jar creation, because there's no documentation and BuildTool itself is tricky, but the application/"fat" jar was started without problems and my application was working without problems.