Rust is a promising programming language for embedded software, providing low-level primitives and performance similar to C/C++ alongside type safety, memory safety, and modern high-level language features. We find naive use of Rust leads to binaries much larger than their C equivalents. For flash-constrained embedded microcontrollers, this is prohibitive. We find four major causes of this growth: monomorphization, inefficient derivations, implicit data structures, and missing compiler optimizations. We present a set of embedded Rust programming principles which reduce Rust binary sizes. We apply these principles to an industrial Rust firmware application, reducing size by 76kB (19%), and an open source Rust OS kernel binary, reducing size by 23kB (26%). We explore compiler optimizations that could further shrink embedded Rust.