I've just built the latest cargo (d6e1726) on ARMv7 Linux, and a fully stripped, dynamically linked, wholly LTO'ed binary optimized for size (opt-level=z) weighs at:
4024204 bytes (3.8MiB) 
MIR,via -Z orbit, brings the size down even further to 3.2 MiB, while also using -C panic=abort, further still, to 2.8 MiB's. (ffa147d)
What's the smallest on your platform?