> Compacting reduces memory usage
No, it reduces memory fragmentation, which is why it's called compacting and not compression.
I do agree that the lack of value types is a big contributor to why Java uses so much memory. But it's not a server tuning thing that makes the JVM lean memory heavy.
The JVM uses moving collectors and that is the big reason why it prefers having so much memory available. Requesting and freeing memory blocks from the OS is an expensive operation which the JVM avoids by grabbing very large blocks of memory all at once. If you have a JVM with 75% old gen and 25% new gen, half that new gen will always be empty because the JVM during collection moves live data from one side of the new gen to the next. And while it does that, it slowly fills up old gen with data.
Even more modern collectors like G1 prefer a large set of empty space because it's moving portions of old gen to empty regions while it does young collection.
As I mentioned, the difference here between the JVM and python or go is that python and go do no moving. They rely heavily on the malloc implementation to handle grabbing right sized blocks from the OS and combating memory fragmentation. But, because they aren't doing any sort of moving, they can get away with having more "right sized" heaps.