JorenCombs
Well, for small processes (below RAM amount) like calculator and such mem_usage usually almost equals vm_size unless they are swapped out due to some other process.
When the process allocates memory above physical limits, VM_size still grows, but mem_size just show amount of physical memory devoted to the process, so up to this point mem_usage <= vm_size.
There is a thing called memory-mapped-files. This doesn't cause vm_size to grow, but it causes memory_usage to grow temporarily while system has some data in-mem which is still not written to the memory-mapped-file disk storage.
I also think that simply reading/writing files might increase per-process memory usage due to caching that data in-mem until they are written to the disk.
So, if we consider disk operations caching as well, mem_usage can be > vm_size.
All in all, I am interested in vm_size on its own