I'm wondering if they are needed, my app has a size of 93 KB, but together with these folders an zipped I got a size of 31 MB.
Is this correct or am I missing something or did something understand wrong?
The size is indeed around 30mb. This is the size of the Java runtime.
If you are distributing an app in a "controlled environment" then you can choose to use Oracle Java 8 and you will then be able to distribute the compiled jar, assuming that Java 8 is already installed on the user computer.