When CPU utilisation is low, the heap can be set much smaller. Many don't know that, so in the next year we'll have the VM do it automatically: https://openjdk.org/jeps/8377305

The amount of memory a Java program uses is whatever the setting is, not how much it "needs", because the need depends on the preference of the CPU/RAM tradeoff. But again, not many understand that, so we're making that automatic.