Lua is strictly worse than MicroPython, I find. Memory usage is worse, and managing programs on it is bad too, although I haven't used it as much as MicroPython.
As for Rust, why do we think that should use significantly less memory than MicroPython or Lua? I've seen some threads with people doing measurements on simple programs and finding Rust up to 10x heavier on RAM than C.
You would need a basic standard library built with the binary when building for xtensa (esp) aswell. So yes, the size of the core parts of the stdlib between c and rust definitely do play a role.
My problem with Lua and MicroPython is that your actual code takes up memory, ie even comments. Micropython can now compile to bytecode and load that, though, so it's better. I've had problems with Lua trying to load one or two hundred lines, though.
