I’ve been working on rust for the cc3200 (Cortex-M4) so similar but not identical to what you’re doing.
My gut tells me that the code size is similar, or perhaps a bit larger. Stack usage can be considerably higher, especially if you do any type of formatted printing output. And you don’t seem to have quite the granularity that you have using a traditional C/Makefile system (i.e. with make buildsystem I can compile most of the system using -Os and the stuff I want to debug with -O0). With cargo, I can’t seem to do that yet.
I’m also currently working on the nrf51822, although my initial foray has been using C and C++. Once I have all of that working, I’m going to take a look at doing the same thing in rust to see how it compares.
It looks like the rust embedded link is primarily for rust on embedded linux. For bare metal stuff, I highly recommend: http://japaric.github.io/copper/ and you should probably look at https://github.com/hackndev/zinc as well.