Regarding your memory usage concern:
Java does not return memory to the OS once it has allocated it (outside of shutting down the Java process). Java's internal garbage collection will automatically recycle the allocated memory.
For this reason, it is recommended to set your initial heap and max heap sizes in the config file to be the same value, so that Ignition allocates the entire expected amount of memory on startup.
The zulu process in task manager reflects the total allocated heap size (plus some overhead), Ignition's status page reflects the amount of the allocated heap actually in use.
Unless you are seeing a trend on the Ignition memory graph where you are slowly consuming all allocated memory and resulting in a gateway hang/shutdown, you are likely okay. Still, it's good to be vigilant and try to avoid common causes of memory leaks.
A good thread to read: