[the8472]

The default sizing policies optimize pause times, throughput and only as tertiary goal for footprint.

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.