Java by default allocates a healthy percentage of available RAM. It releases memory if a large percentage is unused, and it only runs full GC when a large percentage is used. The trick is therefore to decrease the initial max memory. Otherwise you can wind up with 70% usage, a lot of objects that could be GC'd but are not because there is not enough memory pressure to run full GC, and there is not enough unused memory to be able to release RAM back to the OS