Not to mention, in the paper, the lightvm only had an advantage on boot times. Menory usage was marginally worse than docker, even with the unikernel, and debian on lightvm was drastically worse for cpu usage than docker (the unikernel cpu usage was neck and neck with the debian docker contaner).

I could see it being an improvement over other VM control planes, but docker still wins in performance for any equivalant comparisons.

It is still lighter in memory only. CPU is also a relevant thing to compare them.

Not all containers are a "full application stack"

A unikernel can definitely be compared.

I would say that firecracker VMs are not more lightweight than Linux containers.

Linux containers are essentially the separation of Linux processes via various namespaces e.g. mount, cgroup, process, network etc. Because this separation is done by Linux internally there are not that many overheads.

VMs provide a different kind of separation one that is arguably more secure because it is backed up hardware -- each VM thinks it has the whole hardware to itself. When you switch between the VM and the host there is quite a heavyweight context switch (VMEXIT/VMENTER in Intel parlance). It can take a long time compared to just the usual context switch from one Linux container (process) to another host (process) or another Linux container (process).

But coming back to your point, no firecracker VMs are not lighter/lightweight than a Linux container. They are quite heavyweight actually. But the firecracker VMM is probably the most nimble of all VMMs.