(Also, of course, your main server uses "up to" 5 GiB? That's fucking great. Until it goes beyond 64 GiB, who even cares. Work on something that has a worthwhile monetary return..)
Additionally people often tune the JVM to avoid some startup costs which includes fixed heap sizes which prevents the JVM from returning memory to the OS.
On top of that there often are just plain bad programming practices like deserializing large files into memory instead of streaming them. I suspect that rust also attracts more performance-conscious developers that avoid such things. Or maybe it is applied in projects where "throw more hardware at it" is not a cost-effective measure and investing in more developer-hours is appropriate.
I have no doubt that a Rust app will generally be more memory-efficient, but ultimately for server applications that is really not something a lot of people care about because of the incredibly low cost of "just buying more".