I've found that a single database service uses less total resources (especially memory) than running separate DB stacks for each service.

This should indeed be the expected result. Each DB server will have a set amount of overhead from the runtime before data overhead comes in. Ex (made up numbers):

• storage subsystem=256MB
• config subsystem=128MB
• auth subsystem=280MB
• api subsystem=512MB
• user tables=xMB

The subsystem resource usage would be incurred by every instance of the DB server. Additionally, you have platform-level overhead, especially if you are running as VMs or containers as that requires additional resources to coordinate with the kernel, etc.

It's very much like micro-kernels vs monoliths. On the surface a lean micro-kernel seems like it should be more performant since less is happening during kernel time but, the significant increase in operations to perform basic tasks. For example, if storage access was in userspace, an application would need to call back to the kernel to request communication, which would need to call up to the storage driver, then back... and it becomes a death of a thousand cuts. In a monolithic kernel, the application just tells the kernel that it wants to access storage, what mode, and provide either the input or a buffer receive data.