It doesn't sound like you understand how memory Management in Java works. In Java, you have a defined heap size. Java will claim memory in order to support that heap. If you are unhappy about how much memory it is using, you can change the heap size.
Or use ShenadoahGC ;) and it will uncommit heap not in use. Or wait for http://openjdk.java.net/jeps/8204089 to land.
In any case vastly improving idle RSS consumption.