This is normal behavior. There is much more to the JVMs memory usage beyond what’s allocated to the heap - there are other memory regions as well. There are additional tuning options for them, but it’s a complicated subject and if you aren’t actually encountering out of memory issues you have to ask if this is worth the effort to tune it.
Darn it, we’ll never know if the cat gets reunited! It was a fun and unique, if too short, game.