Technically, why are processes in Erlang more efficient than OS threads?

After some more research I found a presentation by Joe Armstrong.

From Erlang - software for a concurrent world (presentation) (at 13 min):

[Erlang] is a concurrent language – by that I mean that threads are part of the programming language, they do not belong to the operating system. That's really what's wrong with programming languages like Java and C++. It's threads aren't in the programming language, threads are something in the operating system – and they inherit all the problems that they have in the operating system. One of the problems is granularity of the memory management system. The memory management in the operating system protects whole pages of memory, so the smallest size that a thread can be is the smallest size of a page. That's actually too big.

If you add more memory to your machine – you have the same number of bits that protects the memory so the granularity of the page tables goes up – you end up using say 64kB for a process you know running in a few hundred bytes.

I think it answers if not all, at least a few of my questions

There are several contributing factors:

1. Erlang processes are not OS processes. They are implemented by the Erlang VM using a lightweight cooperative threading model (preemptive at the Erlang level, but under the control of a cooperatively scheduled runtime). This means that it is much cheaper to switch context, because they only switch at known, controlled points and therefore don't have to save the entire CPU state (normal, SSE and FPU registers, address space mapping, etc.).
2. Erlang processes use dynamically allocated stacks, which start very small and grow as necessary. This permits the spawning of many thousands — even millions — of Erlang processes without sucking up all available RAM.
3. Erlang used to be single-threaded, meaning that there was no requirement to ensure thread-safety between processes. It now supports SMP, but the interaction between Erlang processes on the same scheduler/core is still very lightweight (there are separate run queues per core).

I've implemented coroutines in assembler, and measured performance.

Switching between coroutines, a.k.a. Erlang processes, takes about 16 instructions and 20 nanoseconds on a modern processor. Also, you often know the process you are switching to (example: a process receiving a message in its queue can be implemented as straight hand-off from the calling process to the receiving process) so the scheduler doesn't come into play, making it an O(1) operation.

To switch OS threads, it takes about 500-1000 nanoseconds, because you're calling down to the kernel. The OS thread scheduler might run in O(log(n)) or O(log(log(n))) time, which will start to be noticeable if you have tens of thousands, or even millions of threads.

Therefore, Erlang processes are faster and scale better because both the fundamental operation of switching is faster, and the scheduler runs less often.