I haven't spent time with the code yet, so I can't comment. It shouldn't be the runtime, though, as our runtime is about as big as C or C++'s. My first thought would be that Vec's growth factor may be poor in this instance, but without reading the code, who knows.
> Rust is... ??
Rust statically links everything but glibc by default. Experimental support for musl was added last week.
Rust's runtime is here: https://github.com/rust-lang/rust/tree/master/src/rt and https://github.com/rust-lang/rust/tree/master/src/libstd/rt
As you can see, it's very, very small. It mostly handles things like unwinding, at_exit handlers, and the like.